[Trivial Fix] Replace Chinese punctuation with English punctuation

Curly quotes(Chinese punctuation) usually input from Chinese input method.
When read from english context, it makes some confusion.

Change-Id: I21120d8bc7b4702e8d89ba0663259b5a7c5bfa11

CONTRIBUTING.rst

@@ -79,7 +79,7 @@ When submitting an issue, or working on an issue please ensure the following cri
     * The description clearly states or describes the original problem or root cause of the problem.
     * Include historical information on how the problem was identified.
     * Any relevant logs are included.
-    * If the issue is a bug that needs fixing in a branch other than Master, add the ‘backport potential’ tag TO THE ISSUE (not the PR).
+    * If the issue is a bug that needs fixing in a branch other than Master, add the 'backport potential' tag TO THE ISSUE (not the PR).
     * The provided information should be totally self-contained. External access to web services/sites should not be needed.
     * If the issue is needed for a hotfix release, add the 'expedite' label.
     * Steps to reproduce the problem if possible.