Tweak formatting of PR message template (#3828)

.github/PULL_REQUEST_TEMPLATE.md

 !!! 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
-
-Where: [X.Y] is the branch name, for example [3.6]
+```
+[X.Y] <title from the original PR> (GH-NNNN)
+```
+Where: [X.Y] is the branch name, e.g. [3.6].
 
 GH-NNNN refers to the PR number from `master`.