Update reference-caveats-problem.js (#1626)

Added MyContext for clarity.

examples/context/reference-caveats-problem.js

@@ -2,9 +2,9 @@ class App extends React.Component {
   render() {
     // highlight-range{2}
     return (
-      <Provider value={{something: 'something'}}>
+      <MyContext.Provider value={{something: 'something'}}>
         <Toolbar />
-      </Provider>
+      </MyContext.Provider>
     );
   }
 }