github actions: check whitespace against the main branch

This check was comparing with the opus-ng branch, which does show
some newer files with trailing whitespace, but for pull requests
ends up showing issues from unrelated code. Instead, compare against
the current development branch for more specific feedback.

Signed-off-by: Jean-Marc Valin <jeanmarcv@google.com>

Author: rillian

.github/workflows/repository.yml

@@ -12,4 +12,4 @@ jobs:
           fetch-depth: 0
       - name: Check Whitespaces
         run: |
-          git diff-tree --check origin/opus-ng HEAD
+          git diff-tree --check origin/main HEAD