Scope sponsors-maintainers-man-completions.yml to master branch

This came up when I started pushing some branches directly to the
brew repo instead of using a fork (probably a bad idea in hindsight).

As a result of that, the ever friendly brew test bot took it upon
itself to update the maintainers on each of the branches I'd updated
recently. This was not actually very helpful. We really only need
to run this workflow when there are pushes to the master branch.

.github/workflows/sponsors-maintainers-man-completions.yml

@@ -3,7 +3,7 @@ name: Update sponsors, maintainers, manpage and completions
 on:
   push:
     branches:
-      - "**"
+      - master
     paths:
       - .github/workflows/sponsors-maintainers-man-completions.yml
       - README.md