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.

src/comments.md

diff --git a/src/comments.md b/src/comments.md
index bf09389..44ef06f 100644
--- a/src/comments.md
+++ b/src/comments.md
@@ -35,7 +35,7 @@
 OUTER_BLOCK_DOC ->
     `/**` ![`*` `/`]
       ^
-      ( ~[`*` CR] | NESTED_BLOCK_DOC_COMMENT )
+      ~[`*` CR]
       ( NESTED_BLOCK_DOC_COMMENT | DOC_BLOCK_CHAR )*
     `*/`