Update PULL_REQUEST_TEMPLATE.md (facebook#19813)

Summary:
Use colon after Test Plan to ensure the plan is copied to Facebook's internal Phabricator instance successfully.
Closes facebook#19813

Differential Revision: D8530181

Pulled By: hramos

fbshipit-source-id: 83be6ecaca54a3c6fd4e47b08b8632976f5635aa

Author: hramos

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,31 +1,15 @@
 Thank you for sending the PR! We appreciate you spending the time to work on these changes. 
 Help us understand your motivation by explaining why you decided to make this change.
 
-<!-- 
-  Required: Write your motivation here.
-  If this PR fixes an issue, type "Fixes #issueNumber" to automatically close the issue when the PR is merged.
--->
-
-## Test Plan
-
-<!-- 
-  Required: Write your test plan here. If you changed any code, please provide us with 
-  clear instructions on how you verified your changes work. Bonus points for screenshots and videos! 
--->
+If this PR fixes an issue, type "Fixes #issueNumber" to automatically close the issue when the PR is merged.
 
-## Related PRs
+Test Plan:
+----------
+Write your test plan here. If you changed any code, please provide us with clear instructions on how you verified your changes work. Bonus points for screenshots and videos!
 
-<!-- 
-  Does this PR require a documentation change? 
-  Create a PR at https://github.com/facebook/react-native-website and add a link to it here.
--->
-
-## Release Notes
-
-<!-- 
-  Required. 
-  Help reviewers and the release process by writing your own release notes. See below for an example.
--->
+Release Notes:
+--------------
+Help reviewers and the release process by writing your own release notes. See below for an example.
 
 [CATEGORY] [TYPE] [LOCATION] - Message