Replace last instance of "Heroku" mentioned (#157)

Looks like one line was missed in 3060de0a90

src/intro.tsx

@@ -94,7 +94,7 @@ function Contents() {
           ' cross-site file loading in the browser (CORS).'
         }
         values={{
-          link: <a href="https://topola-cors.herokuapp.com/">cors-anywhere</a>,
+          link: <a href="https://topola-cors-server.up.railway.app/">cors-anywhere</a>,
         }}
       />