Update the Pull_request_template.md file (GH-5841)

Changed the template to be inside a comment tag.

.github/PULL_REQUEST_TEMPLATE.md

-!!! If this is a backport PR (PR made against branches other than `master`),
+<!--
+Thanks for your contribution!
+Please read this comment in its entirety. It's quite important.
+
+# Pull Request title
+
+It should be in the following format:
+```
+bpo-NNNN: Summary of the changes made
+```
+Where: bpo-NNNN refers to the issue number in the https://bugs.python.org.
+If your PR is for issue 12345 in the bug tracker, then the bpo number is bpo-12345.
+
+Most PRs will require an issue number. Trivial changes, like fixing a typo, do not need an issue.
+
+# Backport Pull Request title
+
+If this is a backport PR (PR made against branches other than `master`),
 please ensure that the PR title is in the following format:
 ```
 [X.Y] <title from the original PR> (GH-NNNN)
@@ -7,4 +24,4 @@ Where: [X.Y] is the branch name, e.g. [3.6].
 
 GH-NNNN refers to the PR number from `master`.
 
-PLEASE: Remove this headline!!!
+-->