Fix doc typo in argument_matchers.rb

lib/rspec/mocks/argument_matchers.rb

@@ -356,7 +356,7 @@ def description
         begin
           object.class.name.include?(matcher_namespace)
         rescue NoMethodError
-          # Some objects, like BasicObject, don't implemented standard
+          # Some objects, like BasicObject, don't implement standard
           # reflection methods.
           false
         end