Further update CONTRIBUTING

Tweak to the wording on merge commits.
Reviewed-by: NRich Salz <rsalz@openssl.org>
Reviewed-by: NRichard Levitte <levitte@openssl.org>

CONTRIBUTING

@@ -57,8 +57,8 @@ the acceptance and review process faster:
 
     3.  Patches should be as current as possible.  When using GitHub, please
     expect to have to rebase and update often. Note that we do not accept merge
-    commits, so please avoid these in any pull request. You will be asked to
-    remove them before a patch is considered acceptable.
+    commits. You will be asked to remove them before a patch is considered
+    acceptable.
 
     4.  Patches should follow our coding style (see
     https://www.openssl.org/policies/codingstyle.html) and compile without