fix: replace should with must

Author: gajus

CONTRIBUTING.md

@@ -47,6 +47,6 @@ Run with `npm run lint`.
   * Use [./.README/rules/require-valid-file-annotation.md](./.README/rules/require-valid-file-annotation.md) as a template.
   * Ensure that rule documentation document includes `<!-- assertions spaceAfterTypeColon -->` declaration.
 1. Update [./.README/README.md](/.README/README.md) to include the new rule.
-1. Run `npm run create-readme` to generate the new `README.md` (you should be on `master` branch for this command to work)
+1. Run `npm run create-readme` to generate the new `README.md` (you must be on `master` branch for this command to work)
 
 Note: Sections "The following patterns are considered problems:" and "The following patterns are not considered problems:" are **generated automatically** using the test cases.