change param name to improve documentation

The keys of the error messages are actually attribute names. It makes
the documentation easier to understand:

    # Returns +true+ if the error messages include an error for the given
    # +attribute+, +false+ otherwise.
    #
    #   person.errors.messages # => { :name => ["can not be nil"] }
    #   person.errors.include?(:name) # => true
    #   person.errors.include?(:age)  # => false
    def include?(attribute)
      (v = messages[attribute]) && v.any?
    end

activemodel/lib/active_model/errors.rb

@@ -87,8 +87,8 @@ def clear
     end
 
     # Do the error messages include an error with key +error+?
-    def include?(error)
-      (v = messages[error]) && v.any?
+    def include?(attribute)
+      (v = messages[attribute]) && v.any?
     end
     alias :has_key? :include?