Fixing ext ID (#383)

Extension was renamed...fixing.

.devcontainer/devcontainer.json

@@ -1,9 +1,9 @@
 {  
     "extensions": [
-        "ms-vscode.csharp",
+        "ms-dotnettools.csharp",
         "formulahendry.dotnet-test-explorer",
         "ms-vscode.vscode-node-azure-pack",
         "ms-kubernetes-tools.vscode-kubernetes-tools",
         "redhat.vscode-yaml"
     ]
-  }
+  }