fix build error

Author: harsh62

src/pages/[platform]/build-a-backend/auth/sign-in-with-web-ui/index.mdx

@@ -134,7 +134,7 @@ RxAmplify.Auth.signInWithWebUI(this)
 
  When configuring social sign-in, it's important to exercise caution when designating attributes as "required." Different social identity providers have varied scopes in terms of the information they respond back to Cognito with.  User pool attributes that are initially set up as "required" cannot be changed later, and may require you to migrate the users or create a new user pool.  
 
-</Callout>.
+</Callout>
 
 For a full example, please follow the [project setup walkthrough](/[platform]/start/quickstart/).