fix link in CODEOWNERS

It seems that our local Gitlab no longer has documentation for the
`CODEOWNERS` file, but the master documentation still does. Use
that instead.

CODEOWNERS

 # Confused about what this is? See
-# https://gitlab.haskell.org/help/user/project/code_owners
+# https://docs.gitlab.com/ee/user/project/codeowners/
 
 # Catch-all
 * @bgamari