improve the pull request template

to simplify the mandatory summary specification

.github/pull_request_template.md

@@ -28,6 +28,7 @@ Example: ```SUMMARY: Content "Adds new mutation category 'Mouse'"```
 
 See the [Changelog Guidelines](https://github.com/CleverRaven/Cataclysm-DDA/blob/master/doc/CHANGELOG_GUIDELINES.md) for explanations of the categories.
 -->
+```SUMMARY: Content "changeme"```
 
 #### Purpose of change
 <!--