Added updating nested context default value example (#780)

* added context default value example

* Implement suggested change

* noop function as default value to createContext

examples/context/updating-nested-context-context.js

@@ -0,0 +1,8 @@
+// highlight-range{1-2}
+// Make sure the shape of the default value passed to
+// createContext matches the shape that the consumers expect!
+// highlight-range{2-3}
+export const ThemeContext = React.createContext({
+  theme: themes.dark,
+  toggleTheme: () => {},
+});