compiler: fix stale comment in lexer

Remove stale comment in code that accepts "go:nowritebarrier"
pragma. Update comment for "go:nowritebarrierrec".

Change-Id: Ib278c3f313f810c9cbb00ea6cdb7b267a7303323
Reviewed-on: https://go-review.googlesource.com/46416
Reviewed-by: Ian Lance Taylor <iant@golang.org>
1 file changed