Correct the doc about that approver is not expected to merge PR (#469)

Author: ThomsonTan

CONTRIBUTING.md

@@ -116,7 +116,7 @@ A PR is considered to be **ready to merge** when:
 * Trivial changes (typo, cosmetic, doc, etc.) don't have to wait for one day.
 * Urgent fixes can take exceptions as long as it has been actively communicated.
 
-Any Approver / Maintainer can merge the PR once it is **ready to merge**.
+Any Maintainer can merge the PR once it is **ready to merge**.
 
 If a PR has been stuck (e.g. there are lots of debates and people couldn't agree on each other), the owner should try to get people aligned by: