Update linking-a-pull-request-to-an-issue.md

Updated with requested changes

content/github/managing-your-work-on-github/linking-a-pull-request-to-an-issue.md

@@ -43,7 +43,7 @@ You can manually link up to ten issues to each pull request. The issue and pull
 
 ### Linking a pull request to an issue using a keyword
 
-You can link a pull request to an issue by using a supported keyword in the pull request's description or in a commit message. 
+You can link a pull request to an issue by using a supported keyword in the pull request's description or in a commit message (please note that the PR must be on the default branch). 
 
 * close
 * closes