chore(readme): userid explain

Author: SSebo

README.md

@@ -57,8 +57,8 @@ After you install and import the SDK, create a single, shared instance of the Fe
 NPM:
 
 ```js
-const uniqueUserId = /* uniqueUserId */;
-const user = new FPUser(uniqueUserId);
+const userId = /* unique user id in your business logic */;
+const user = new FPUser(userId);
 user.with("userId", /* userId */);
 
 let remoteUrl = "https://featureprobe.io/server";

example/index.html

@@ -66,7 +66,7 @@ <h3>json type</h3>
 
     </div>
     <script>
-      const user = new featureProbe.FPUser("unique_user_key");
+      const user = new featureProbe.FPUser("unique_user_id");
       user.with("userId", "1234567890");
 
       const fpClient = new featureProbe.FeatureProbe({