Changed the description of 'have_attribute_matcher' from 'has' to 'have'

I changed the description of 'have_attribute_matcher' from 'has' to 'have' to be more consistent with descriptions used by other Rspec matchers.  It also now reads as proper english when you view the spec output with flag '--format documentation'.

BEFORE:
it { should have_attribute(:email) }
=> should has attribute named email

AFTER:
it { should have_attribute(:email) }
=> should have attribute named email