Drop --diff from docbook-cs as per php/docbook-cs#33

Author: jordikroon

.github/workflows/docbook-cs.yaml

@@ -93,7 +93,6 @@ jobs:
           set -o pipefail
           git diff "$BASE_SHA"...HEAD | php ../docbook-cs/bin/docbook-cs \
             --report=checkstyle \
-            --diff \
             --no-colors > docbook-report.xml
 
       - name: "Upload report"