Fix typo

frontend/README.md

 ### Prerequisites
 
 In order to run this application, npm (>= 6.9.0) needs to be installed. As the application
-will authenticate it's users using OpenID Connect, an OpenID Connect Provider and an OpenID Client
+will authenticate its users using OpenID Connect, an OpenID Connect Provider and an OpenID Client
 needs to be set up.
 This application was developed to be used with [an OpenID Connect single sign on system](https://open.greenhost.net/openappstack/single-sign-on).