doc: `npm ls react` is a better advice (#7513)

Refs Must Have Owner Warning
`npm ls react` is a better advice than `npm ls | grep react`

warnings/refs-must-have-owner.md

@@ -24,4 +24,4 @@ Only a ReactOwner can have refs. This usually means that you're trying to add a
 
 Bower does a good job of deduplicating dependencies, but NPM does not. If you aren't doing anything (fancy) with refs, there is a good chance that the problem is not with your refs, but rather an issue with having multiple copies of React loaded into your project. Sometimes, when you pull in a third-party module via npm, you will get a duplicate copy of the dependency library, and this can create problems.
 
-If you are using npm... `npm ls` or `npm ls | grep react` might help illuminate.
+If you are using npm... `npm ls` or `npm ls react` might help illuminate.