Fix incorrect index in MSTest template (#48168)

Author: nohwnd

template_feed/Microsoft.DotNet.Common.ProjectTemplates.10.0/content/MSTest-CSharp/.template.config/template.json

@@ -234,7 +234,7 @@
       "actionId": "84C0DA21-51C8-4541-9940-6CA19AF04EE6",
       "id": "openInEditor",
       "args": {
-        "files": "2"
+        "files": "1"
       },
       "continueOnError": true
     }