docs(ngAnimate): change "&#64" to "@" symbol

Request Type: docs

How to reproduce:
See http://docs.angularjs.org/api/ngAnimate

Component(s): ngAnimate

Impact: small

Complexity: small

This issue is related to:

**Detailed Description:**

&#64 was not rendered as an "@" symbol (even after adding the missing semicolon). Replacing this with an actual "@" symbol.
Closes #6822.