Internal: Updated the GitHub PR template because all packages share the same...

Internal: Updated the GitHub PR template because all packages share the same issue tracker now (see ckeditor/ckeditor5#5576).

.github/PULL_REQUEST_TEMPLATE.md

 ### Suggested merge commit message ([convention](https://github.com/ckeditor/ckeditor5-design/wiki/Git-commit-message-convention))
 
-Type: Message. Closes #000.
+Type: Message. Closes ckeditor/ckeditor5#000.
 
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