Comma-fix and clarity

Attempting to improve clarity and grammar slightly, while inserting a needed comma in the second sentence.

Author: shoesandsocks

docs/rules/no-string-refs.md

@@ -1,6 +1,6 @@
 # Prevent using string references (no-string-refs)
 
-Currently, two ways are supported by React to refer to components. The first one, providing a string identifier is considered legacy in the official documentation. Referring to components by setting a property on the `this` object in the reference callback is preferred.
+Currently, two ways are supported by React to refer to components. The first way, providing a string identifier, is now considered legacy in the official documentation. The documentation now prefers a second method -- referring to components by setting a property on the `this` object in the reference callback.
 
 ## Rule Details