docs: fix code snippet formatting and highlight lines

Correct a mismatched line highlight in the documentation examples
and convert tags to the preferred self-closing format for consistency.

Author: MeAkib

adev/src/content/guide/components/outputs.md

@@ -4,7 +4,7 @@ TIP: This guide assumes you've already read the [Essentials Guide](essentials).
 
 Angular components can define custom events by assigning a property to the `output` function:
 
-```ts {highlight:[3]}
+```ts {highlight:[5]}
 @Component({
   /*...*/
 })

adev/src/content/guide/elements.md

@@ -39,7 +39,7 @@ The `createCustomElement()` function converts a component into a class that can
 After you register your configured class with the browser's custom-element registry, use the new element just like a built-in HTML element in content that you add directly into the DOM:
 
 ```html
-<my-popup message="Use Angular!"></my-popup>
+<my-popup message="Use Angular!" />
 ```
 
 When your custom element is placed on a page, the browser creates an instance of the registered class and adds it to the DOM.