Code of conduct: fix punctuation

Reported in https://wiki.archlinux.org/index.php/Talk:Code_of_conduct#Typo_in_Cross-posting_section .

code-of-conduct.md

@@ -331,7 +331,7 @@ unwillingness to point out the obvious.
 
 Cross-posting is posting the same question multiple times in different subfora (for example,
 posting in both Newbie Corner and Installation), or posting slight variants of the question in the
-same or different subfora . This is a waste of resources and is not permitted. Any cross-posted
+same or different subfora. This is a waste of resources and is not permitted. Any cross-posted
 topic will be immediately locked and marked for deletion.
 
 #### Thread hijacking