nixos/taskserver/helper: Fix docstring of add_user

We have already revamped the CLI subcommands in commit
e2383b84f88e0e7d35f6a3a846b54c69e3bee6ee.

This was just an artifact that was left because of this.

Signed-off-by: aszlig <[e-mail]>

nixos/modules/services/misc/taskserver/helper-tool.py

···
     and print the key of the new user.
 
     The client certificate along with it's public key can be shown via the
-    'export-user' subcommand.
+    'user export' subcommand.
     """
     userobj = organisation.add_user(user)
     if userobj is None: