rust: alloc: fix assert in Vec::reserve doc test

The assert in the doctest used `>= 10`, which only checks that the
capacity can hold `additional` elements, ignoring the existing length
of `v`. The correct check should ensure there is room for `additional`
*extra* elements on top of what is already in the vector.

Fix the assert to use `>= v.len() + 10` so the example accurately
reflects the actual semantics of the function.

Reported-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Closes: https://lore.kernel.org/rust-for-linux/CANiq72nkXWhjK9iFRrhGtkMZGsvNE_zVsu4JnxaFRfxWL7RRdg@mail.gmail.com/
Fixes: 2aac4cd ("rust: alloc: implement kernel `Vec` type")
Signed-off-by: Hsiu Che Yu <yu.whisper.personal@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Link: https://patch.msgid.link/20260427-doctest-kvec-reserve-v1-1-0623abcd9c2e@gmail.com
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

Author: Hsiu Che Yu

rust/kernel/alloc/kvec.rs

@@ -614,7 +614,7 @@ where
     ///
     /// v.reserve(10, GFP_KERNEL)?;
     /// let cap = v.capacity();
-    /// assert!(cap >= 10);
+    /// assert!(cap >= v.len() + 10);
     ///
     /// v.reserve(10, GFP_KERNEL)?;
     /// let new_cap = v.capacity();