PULL_REQUEST_TEMPLATE: clarify that issue number needs to be replaced (commons-app#3599)

Some contributors seem to be missing the fact that they have to mention
the issue number right after `Fixes ` to ensure the corresponding issue
gets closed[1] when the PR gets merged.

Clarify this by using a better phrase that clearly states that they
have to replace the placeholder text with the issue number.

[1]: https://help.github.com/en/github/managing-your-work-on-github/linking-a-pull-request-to-an-issue#linking-a-pull-request-to-an-issue-using-a-keyword

Author: sivaraam

PULL_REQUEST_TEMPLATE.md

@@ -1,6 +1,6 @@
 **Description (required)**
 
-Fixes #{GitHub issue number}
+Fixes #INSERT_ISSUE_NUMBER_HERE
 
 What changes did you make and why?