docs: unqualify mobx version for react usage (#1380)

Per #1189 (comment) the latest version works now, and the currently-specified version doesn't.

README.md

@@ -138,7 +138,7 @@ For npm:
 
 Install peer dependencies required by ReDoc if you don't have them installed already:
 
-    npm i react react-dom mobx@^4.2.0 styled-components core-js
+    npm i react react-dom mobx styled-components core-js
 
 Import `RedocStandalone` component from 'redoc' module: