Rewrote the last fun example

The line above the example says: 
> It is important to note that the argument to function fun() must be a single-quoted, constant literal string representing a valid function name. For example:

I did not understand the example at all... Nobody expected `func($str)` to work and it has nothing to do with the line above. `$func($str)` will work, but I believe this was not the purpose of the example.

I changed the example to illustrate a situation where we are not using a "constant literal string". I think this was the intention of the example.