docs: Update injectScript docs to match example from examples repo (#1240)

Co-authored-by: Aaron <aaronklinker1@gmail.com>

docs/guide/essentials/content-scripts.md

@@ -548,16 +548,21 @@ To use `injectScript`, we need two entrypoints, one content script and one unlis
 ```ts
 // entrypoints/example-main-world.ts
 export default defineUnlistedScript(() => {
-  console.log('Hello from the main world!');
+  console.log('Hello from the main world');
 });
 ```
 
 ```ts
 // entrypoints/example.content.ts
-export default defineContentScript(async () => {
-  await injectScript('/example-main-world.js', {
-    keepInDom: true,
-  });
+export default defineContentScript({
+  matches: ['*://*/*'],
+  async main() {
+    console.log('Injecting script...');
+    await injectScript('/example-main-world.js', {
+      keepInDom: true,
+    });
+    console.log('Done!');
+  },
 });
 ```