update with existing .openpublishing.publish.config.json (#104)

Co-authored-by: Ying Hua <herohua@users.noreply.github.com>

.openpublishing.publish.config.json

@@ -46,7 +46,7 @@
     {
       "path_to_root": "_themes",
       "url": "https://github.com/Microsoft/templates.docs.msft",
-      "branch": "master",
+      "branch": "main",
       "branch_mapping": {}
     }
   ],
@@ -87,4 +87,4 @@
   "docs_build_engine": {
     "name": "docfx_v3"
   }
-}
+}