Change master references with main (editor-bootstrap#401)

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,7 +1,7 @@
 Please check the following:
 
-1. Make sure you are targeting the `master` branch, pull requests on release branches are only allowed for bug fixes.
-2. Read contributing guidelines: https://github.com/editor-bootstrap/vim-bootstrap/blob/master/.github/CONTRIBUTING.md
+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/editor-bootstrap/vim-bootstrap/blob/main/.github/CONTRIBUTING.md
 3. Describe what your pull request does and which issue you're targeting (if any)
 
 **You MUST delete the content above including this line before posting, otherwise your pull request will be invalid.**