Add more information about multiple Google scopes

Information from #126 (comment)

flutter_web_auth_2/README.md

@@ -80,7 +80,7 @@ final response = await http.post(url, body: {
 final accessToken = jsonDecode(response.body)['access_token'] as String;
 ```
 
-**Note:** To use multiple scopes with Google, you need to encode them as a single string, separated by spaces. For example, `scope: 'email https://www.googleapis.com/auth/userinfo.profile'`. Here is [a list of all supported scopes](https://developers.google.com/identity/protocols/oauth2/scopes).
+**Note:** To use multiple scopes with Google, you need to encode them as a single string, separated by spaces (`%20`). For example, `scope: 'email https://www.googleapis.com/auth/userinfo.profile'`. Here is [a list of all supported scopes](https://developers.google.com/identity/protocols/oauth2/scopes).
 
 ## Migration