Provide exception for listing an issue. (#137092)

Latest example of an issue about a typo: https://github.com/flutter/flutter/issues/137080. I don't think these are helpful, so we shouldn't be encouraging people to file these in order to submit a PR to fix 'em.

.github/PULL_REQUEST_TEMPLATE.md

 *Replace this paragraph with a description of what this PR is changing or adding, and why. Consider including before/after screenshots.*
 
-*List which issues are fixed by this PR. You must list at least one issue.*
+*List which issues are fixed by this PR. You must list at least one issue. An issue is not required if the PR fixes something trivial like a typo.*
 
 *If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*