Tweak variable name.

tailbone/views/users.py

@@ -169,10 +169,10 @@ class UserCrud(CrudView):
                 ])
 
         if self.creating:
-            def valid_username(value, field):
+            def unique_username(value, field):
                 if Session.query(User).filter_by(username=value).count():
                     raise ValidationError("Username must be unique.")
-            fs.username.set(validate=valid_username)
+            fs.username.set(validate=unique_username)
 
         if self.readonly:
             del fs.password