Update introduction.md

changed postman test link to reflect change in branch name

documentation/docs/specs/backend-specs/introduction.md

@@ -7,6 +7,6 @@ sidebar_position: 1
 
 All backend implementations need to adhere to our [API spec](https://github.com/gothinkster/realworld/tree/master/api).
 
-For your convenience, we have a [Postman collection](https://github.com/gothinkster/realworld/blob/master/api/Conduit.postman_collection.json) that you can use to test your API endpoints as you build your app.
+For your convenience, we have a [Postman collection](https://github.com/gothinkster/realworld/blob/main/api/Conduit.postman_collection.json) that you can use to test your API endpoints as you build your app.
 
 Our [starter kit](https://github.com/gothinkster/realworld-starter-kit) includes [references to the API specs & testing](https://github.com/gothinkster/realworld-starter-kit/blob/master/BACKEND_INSTRUCTIONS.md) required for creating a new backend.