docs(common): remove h1s from API docs

Only one h1 is allowed per document, and this is provided by the template.
So we cannot have any h1 tags (or `#` markdown shorthand) in any API docs.

Closes #16193

packages/common/src/directives/ng_template_outlet.ts

@@ -26,7 +26,7 @@ import {Directive, EmbeddedViewRef, Input, OnChanges, SimpleChanges, TemplateRef
  *
  * Note: using the key `$implicit` in the context object will set it's value as default.
  *
- * # Example
+ * ## Example
  *
  * {@example common/ngTemplateOutlet/ts/module.ts region='NgTemplateOutlet'}
  *