If you're using the HTTrack GUI, click on the "Set Options" button when setting
up the project. In the "Browser ID" tab, there are several options - one of
which is the "HTML footer" field. By default, it says:

<!-- Mirrored from %s%s by HTTrack Website Copier/3.x [XR&CO'2014], %s -->

where the %s's are filled in with variables when it's run. You can simply
change the dropdown to the "(none)" option, and HTTrack will not add any
footers to your content, and so your content will not contain the word
"HTTrack" at all.