Using Angular markup like `{{hash}}` in a `src` attribute doesn't
work right: The browser will fetch from the URL with the literal
text `{{hash}}` until Angular replaces the expression inside
`{{hash}}`. The `ngSrc` directive solves this problem.
The buggy way to write it:
```html
<img src="http://www.gravatar.com/avatar/{{hash}}" alt="Description"/>
```
The correct way to write it:
```html
<img ng-src="http://www.gravatar.com/avatar/{{hash}}" alt="Description" />
```