clippy: docs formatting

This is the most opinonated suggestion in this sequence of patches.
Probably IDK if would be an option disable this check, but for now
this commit propose a formatting fix to make clippy happy.

```
error: doc list item overindented
  --> lightning-types/src/features.rs:62:5
   |
62 | //!    onion.
   |     ^^^ help: try using `  ` (2 spaces)
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#doc_overindented_list_items
   = note: `-D clippy::doc-overindented-list-items` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(clippy::doc_overindented_list_items)]`

error: doc list item overindented
  --> lightning-types/src/features.rs:63:5
   |
63 | //!    (see [BOLT-11](https://github.com/lightning/bolts/blob/master/11-payment-encoding.md) for
   |     ^^^ help: try using `  ` (2 spaces)
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#doc_overindented_list_items

error: doc list item overindented
  --> lightning-types/src/features.rs:64:5
   |
64 | //!    more).
   |     ^^^ help: try using `  ` (2 spaces)
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#doc_overindented_list_items

error: doc list item overindented
  --> lightning-types/src/features.rs:66:5
   |
66 | //!    (see
   |     ^^^ help: try using `  ` (2 spaces)
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#doc_overindented_list_items

error: doc list item overindented
  --> lightning-types/src/features.rs:67:5
   |
67 | //!    [BOLT-2](https://github.com/lightning/bolts/blob/master/02-peer-protocol.md#the-channel_ready-message)
   |     ^^^ help: try using `  ` (2 spaces)
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#doc_overindented_list_items

error: doc list item overindented
  --> lightning-types/src/features.rs:68:5
   |
68 | //!    for more info).
   |     ^^^ help: try using `  ` (2 spaces)
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#doc_overindented_list_items

error: could not compile `lightning-types` (lib) due to 6 previous errors
```

Suggested-by: @dunxen
Signed-off-by: Vincenzo Palazzo <[email protected]>

Author: vincenzopalazzo

lightning-types/src/features.rs

@@ -59,13 +59,13 @@
 //! - `SCIDPrivacy` - supply channel aliases for routing
 //!   (see [BOLT-2](https://github.com/lightning/bolts/blob/master/02-peer-protocol.md) for more information).
 //! - `PaymentMetadata` - include additional data in invoices which is passed to recipients in the
-//!    onion.
-//!    (see [BOLT-11](https://github.com/lightning/bolts/blob/master/11-payment-encoding.md) for
-//!    more).
+//!   onion.
+//!   (see [BOLT-11](https://github.com/lightning/bolts/blob/master/11-payment-encoding.md) for
+//!   more).
 //! - `ZeroConf` - supports accepting HTLCs and using channels prior to funding confirmation
-//!    (see
-//!    [BOLT-2](https://github.com/lightning/bolts/blob/master/02-peer-protocol.md#the-channel_ready-message)
-//!    for more info).
+//!   (see
+//!   [BOLT-2](https://github.com/lightning/bolts/blob/master/02-peer-protocol.md#the-channel_ready-message)
+//!   for more info).
 //! - `Keysend` - send funds to a node without an invoice
 //!   (see the [`Keysend` feature assignment proposal](https://github.com/lightning/bolts/issues/605#issuecomment-606679798) for more information).
 //! - `Trampoline` - supports receiving and forwarding Trampoline payments