Update pull_request_template.md (#15948)

Branch name
- master -> main

.github/pull_request_template.md

 Please check the following:
 
-1. Make sure you are targeting the `master` branch, pull requests on release branches are only allowed for bug fixes.
+1. Make sure you are targeting the `main` branch, pull requests on release branches are only allowed for bug fixes.
 2. Read contributing guidelines: https://github.com/go-gitea/gitea/blob/master/CONTRIBUTING.md
 3. Describe what your pull request does and which issue you're targeting (if any)