Fix CODEOWNERS for doc/stdlib.

It never made much sense to have the code owners for this directory be
the doc-maintainers while the actual content of the stdlib
documentation is written within the .v files. In practice, this has
just meant that doc-maintainers get a review request when adding or
removing a file in the stdlib, which is a bit absurd.

.github/CODEOWNERS

@@ -124,6 +124,7 @@
 ########## Standard library and plugins ##########
 
 /theories/         @coq/stdlib-maintainers
+/doc/stdlib        @coq/stdlib-maintainers
 
 /theories/Classes/ @coq/typeclasses-maintainers