Remove unreachable alternative in OUTER_BLOCK_DOC

The only way to match the `NESTED_BLOCK_DOC_COMMENT` rule is for the
string to start with `/`, but we've already ruled that out with a
negative lookahead, making this alternative dead.  Let's remove it.

Author: traviscross

src/comments.md

@@ -35,7 +35,7 @@ OUTER_LINE_DOC ->
 OUTER_BLOCK_DOC ->
     `/**` ![`*` `/`]
       ^
-      ( ~[`*` CR] | NESTED_BLOCK_DOC_COMMENT )
+      ~[`*` CR]
       ( NESTED_BLOCK_DOC_COMMENT | DOC_BLOCK_CHAR )*
     `*/`