Trivial: Make user categories consistent

On the content, user categories are defined as service account and
normal user. However regular user is written at one place instead
of normal user. This replaces the regular user with normal user
for the consistency.

content/en/docs/reference/access-authn-authz/authentication.md

@@ -23,7 +23,7 @@ by Kubernetes, and normal users.
 Normal users are assumed to be managed by an outside, independent service. An
 admin distributing private keys, a user store like Keystone or Google Accounts,
 even a file with a list of usernames and passwords. In this regard, _Kubernetes
-does not have objects which represent normal user accounts._ Regular users
+does not have objects which represent normal user accounts._ Normal users
 cannot be added to a cluster through an API call.
 
 In contrast, service accounts are users managed by the Kubernetes API. They are