TopUp.images_path small incorrectness in documentation

[rainerfrey]

According to the documentation page, the default is
TopUp.images_path="/images/top_up/";

In current release it is actually
TopUp.images_path="images/top_up/";

without leading slash, which causes errors when TopUp is used in nested subdirectory pages when one has put the images directory into the webroot.

I'd prefer the default to be changed in the code, so that the same topup image directory can be used in different supdirectories of a web project. But at the very least the documentation should match the code.