Fix deprecation warning

Author: zetachang

spec/react/dsl_spec.rb

@@ -172,7 +172,7 @@ def render
         _undefined_method
       end
     end
-    expect { React.render_to_static_markup(React.create_element(Foo)) }
+    expect { React::Server.render_to_static_markup(React.create_element(Foo)) }
       .to raise_error(NoMethodError)
   end