Use single quotes in the Git commit command in AppVeyor.

appveyor.yml

@@ -56,7 +56,7 @@ on_success:
   - ps: Add-Content "$env:USERPROFILE\.git-credentials" "https://$($env:GITHUB_ACCESS_TOKEN):x-oauth-basic@github.com`n"
   - git remote set-url origin https://github.com/mono/CppSharp.git
   - '%LIB_PATH%\CppSharp.Parser.Gen.exe'
-  - git commit -a -m "CI: Re-generated the parser bindings"
+  - 'git commit -a -m "CI: Re-generated the parser bindings"'
   - git push
 
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