docs(guide/forms): remove unnecessary controller reference

Request Type: docs

How to reproduce: Go to http://docs.angularjs.org/guide/forms, then Custom Validation -> the example does not work at all

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**Detailed Description:**

The controller reference was breaking the custom validation example. Interestíngly, in 1.13 the example worked on the docs page, but was broken in the plunker example.

**Other Comments:**

Closes #6525