Encapsulate header-theme example plugin in a Gerrit.install call

To make it more clear that you still need Gerrit.install for DOM styling.

Release-Notes: skip
Bug: Issue 16031
Change-Id: Ib0970d66adfa3f6d7924d3fc582d7d24af188d30
diff --git a/Documentation/pg-plugin-dev.txt b/Documentation/pg-plugin-dev.txt
index b8a30ee..3f7d90d 100644
--- a/Documentation/pg-plugin-dev.txt
+++ b/Documentation/pg-plugin-dev.txt
@@ -101,16 +101,18 @@
 Just add code like this to your JavaScript plugin:
 
 ``` js
-    const styleEl = document.createElement('style');
-    styleEl.innerHTML = `
-        html {
-          --header-background-color: #c3d9ff;
-        }
-        html.darkTheme {
-          --header-background-color: #c3d9ff90;
-        }
-    `;
-    document.head.appendChild(styleEl);
+Gerrit.install(plugin => {
+  const styleEl = document.createElement('style');
+  styleEl.innerHTML = `
+      html {
+        --header-background-color: #c3d9ff;
+      }
+      html.darkTheme {
+        --header-background-color: #c3d9ff90;
+      }
+  `;
+  document.head.appendChild(styleEl);
+});
 ```
 
 [[high-level-api-concepts]]