Corrected  misspelled method in horizon documentation

In the page:  http://docs.openstack.org/developer/horizon/topics/testing.html had two misspelled assertion method.

asertNoMessages() changed by assertNoMessages(), twice.

Change-Id: Icde7d230e57701eb93ed4f0e705b7737260d7ecf
Fixes: without launchpad bug number.
Merge "Corrected  misspelled method in horizon documentation"