Added updating nested context default value example (#780)

* added context default value example

* Implement suggested change

* noop function as default value to createContext

content/docs/context.md

@@ -97,6 +97,9 @@ A more complex example with dynamic values for the theme:
 
 It is often necessary to update the context from a component that is nested somewhere deeply in the component tree. In this case you can pass a function down through the context to allow consumers to update the context:
 
+**theme-context.js**
+`embed:context/updating-nested-context-context.js`
+
 **theme-toggler-button.js**
 `embed:context/updating-nested-context-theme-toggler-button.js`
 

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: () => {},
+});