Fix build: update windows-code-snippets-pr branch to main

The windows-code-snippets-pr repository branch has been renamed from
'master' to 'main'. Update the dependent repository configuration to
use the correct branch name.

This fixes the build error:
'Failure to clone the repository with branch master'

Author: GrantMeStrength

.openpublishing.publish.config.json

@@ -88,7 +88,7 @@
     {
       "path_to_root": "snippets-windows",
       "url": "https://github.com/MicrosoftDocs/windows-code-snippets-pr",
-      "branch": "master",
+      "branch": "main",
       "branch_mapping": {}
     },
     {