Add --diff to black command in GitHub Action

.github/workflows/ci.yml

@@ -93,7 +93,7 @@ jobs:
       if: matrix.os != 'windows-latest' && matrix.python-version != '2.7'
       run: |
         pip install black
-        black --line-length 79 --check labelme
+        black --line-length 79 --check --diff labelme
 
     - name: Test with pytest
       shell: bash -l {0}