chore(docs): clarify that dataClientToken comes from the server (#375)

Author: gregjopa

README.md

@@ -240,7 +240,8 @@ export default function App() {
         <PayPalScriptProvider
             options={{
                 clientId: "test",
-                dataClientToken: "abc123xyz==",
+                dataClientToken:
+                    "<the data-client-token value generated by your server-side code>",
             }}
         >
             <BraintreePayPalButtons