docs: fixed typo in 'update a user' section from doc (#5806)

# Description
I noticed a typo in the documentation as I was going over the
introductory tutorials and executing the code on my side, so I fixed it.

Closes #5798 

**Type of change**
- Documentation update

**How Has This Been Tested**
- Not applicable

**Checklist**
- All OK.

argilla/docs/how_to_guides/user.md

@@ -220,10 +220,10 @@ client = rg.Argilla(api_url="<api_url>", api_key="<api_key>")
 
 user_to_update = client.users('my_username')
 
-user.username = 'new_username'
-user.first_name = 'new_first_name'
-user.last_name = 'new_last_name'
-user.role = 'admin'
+user_to_update.username = 'new_username'
+user_to_update.first_name = 'new_first_name'
+user_to_update.last_name = 'new_last_name'
+user_to_update.role = 'admin'
 
 updated_user = user_to_update.update()
 ```