Code works, but warning about call_user_func_array() appears

I’m a newbie to WordPress. I’m trying to make custom page that displays contact info for all users.
I use code snippet provided by THIS POST.

It works, but it also shows this warning at the top:

Warning: call_user_func_array() expects parameter 1 to be a valid callback, function ‘yoursite_pre_user_query’ not found or invalid function name in C:xampphtdocsmythemewordpresswp-includesclass-wp-hook.php on line 287

What should I do to resolve this?