docs(list): clarify that md-avatar-icon is for use with icons (angula…

…r#11396)

current example causes confusion that it can be used with images
it also uses an icon set that is not defined in the docs site

Fixes angular#10382

src/components/list/list.js

@@ -195,7 +195,7 @@ function mdListDirective($mdTheming) {
  * When using `<md-icon>` for an avatar, you have to use the `.md-avatar-icon` class.
  * <hljs lang="html">
  *   <md-list-item>
- *     <md-icon class="md-avatar-icon" md-svg-icon="avatars:timothy"></md-icon>
+ *     <md-icon class="md-avatar-icon" md-svg-icon="social:person"></md-icon>
  *     <span>Timothy Kopra</span>
  *   </md-list-item>
  * </hljs>