Improve doc string.

"local instance" didn't really give me the right
sense for what this was testing.

spec/rspec/mocks/any_instance_spec.rb

@@ -295,7 +295,7 @@ class RSpec::SampleRspecTestClass;end
           expect(obj.existing_method).to eq(:existing_method_return_value)
         end
 
-        it "does not remove any stubs on the local instance" do
+        it "does not remove any stubs set directly on an instance" do
           klass.any_instance.stub(:existing_method).and_return(:any_instance_value)
           obj = klass.new
           obj.stub(:existing_method).and_return(:local_method)