try different sed syntax

Author: kinkie

.github/workflows/gen-docs.yaml

@@ -75,7 +75,7 @@ jobs:
       - name: Build programming guide
         if: steps.check_squid_updates.outputs.had_update == 'true'
         run: |
-          sed -i~ 's/PROJECT_NUMBER           = .*/&-${{ steps.check_squid_updates.outputs.current}}/' doxyfile
+          sed -i~ 's/^PROJECT_NUMBER.*= .*/PROJECT_NUMBER = master\/${{ steps.check_squid_updates.outputs.current}}/' doxyfile
           doxygen doxyfile
 
       - name: update self