Fix devcontainer.json format (#23715)

.devcontainer/devcontainer.json

@@ -4,6 +4,10 @@
   "service": "app",
   "workspaceFolder": "/mastodon",
 
+  // Configure tool-specific properties.
+  "customizations": {
+    // Configure properties specific to VS Code.
+    "vscode": {
       // Set *default* container specific settings.json values on container create.
       "settings": {},
 
@@ -13,7 +17,9 @@
         "dbaeumer.vscode-eslint",
         "rebornix.Ruby",
         "webben.browserslist"
-  ],
+      ]
+    }
+  },
 
   "features": {
     "ghcr.io/devcontainers/features/sshd:1": {