Rephrase sentence (#4880)

Rephrase sentence referring to the single legacy method of 'UNSAFE_comonentWillMount()' on the Notes section within the 'Mounting' topic.

content/docs/reference-react-component.md

@@ -52,7 +52,7 @@ These methods are called in the following order when an instance of a component
 
 >Note:
 >
->These methods are considered legacy and you should [avoid them](/blog/2018/03/27/update-on-async-rendering.html) in new code:
+>This method is considered legacy and you should [avoid it](/blog/2018/03/27/update-on-async-rendering.html) in new code:
 >
 >- [`UNSAFE_componentWillMount()`](#unsafe_componentwillmount)