Fix regexp in help-do-arg-highlight

Fix regexp in help-do-arg-highlight

* lisp/help-fns.el (help-do-arg-highlight): Fix regexp.  (Bug#65580)

Avoid highlighting (parts of) the function name as arguments in the
output of 'describe-function' when the function has an argument with
the same name.  To achieve this, refine the regular expression that
'help-do-arg-highlight' uses such that it doesn't match anything in
the function name.