Updated the document's structure

doc/development/fe_guide/performance.md

@@ -127,21 +127,21 @@ To use the Vue performance plugin:
 
 1. Import the plugin:
 
-  ```javascript
-  import PerformancePlugin from '~/performance/vue_performance_plugin';
-  ```
+    ```javascript
+    import PerformancePlugin from '~/performance/vue_performance_plugin';
+    ```
 
 1. Use it before initializing your Vue application:
 
-  ```javascript
-  Vue.use(PerformancePlugin, {
-    components: [
-      'IdeTreeList',
-      'FileTree',
-      'RepoEditor',
-    ]
-  });
-  ```
+    ```javascript
+    Vue.use(PerformancePlugin, {
+      components: [
+        'IdeTreeList',
+        'FileTree',
+        'RepoEditor',
+      ]
+    });
+    ```
 
 The plugin accepts the list of components, performance of which should be measured. The components
 should be specified by their `name` option.
@@ -182,7 +182,7 @@ To access stored measurements, you can use either:
   performance.getEntriesByType('measure');
   ```
 
-## Naming convention
+### Naming convention
 
 All the marks and measures should be instantiated with the constants from
 `app/assets/javascripts/performance/constants.js`. When you’re ready to add a new mark’s or