Update code-splitting.md (#556)

Fixed a typo: "A good place **is** to start is with routes" => "A good place to start is with routes"

content/docs/code-splitting.md

@@ -157,7 +157,7 @@ Deciding where in your app to introduce code splitting can be a bit tricky. You
 want to make sure you choose places that will split bundles evenly, but won't
 disrupt the user experience.
 
-A good place is to start is with routes. Most people on the web are used to
+A good place to start is with routes. Most people on the web are used to
 page transitions taking some amount of time to load. You also tend to be
 re-rendering the entire page at once so your users are unlikely to be
 interacting with other elements on the page at the same time.