Improve PR template

.github/pull_request_template.md

@@ -1,5 +1,5 @@
 <!-- Please remember to select the appropriate branch:
 
-For bug or doc fixes pick the oldest branch where the fix applies (e.g. 2.1 or similar if such a branch exists, or 1.10 if it is a critical fix that should be fixed in Composer 1)
+For bug or doc fixes pick the oldest branch where the fix applies (e.g. `2.2` if the 2.2 LTS is affected, `1.10` if it is a critical fix that should be fixed in Composer 1, otherwise `main`)
 
 For new features and everything else, use the main branch. -->