fix link to turnitin settings page

Hi John

I've found that the link to the settings page is broken. For example, on plagiarism/turnitin/settings.php the link "here" goes to plagiarism/turnitin/admin/settings.php?section=modsettingturnitintooltwo" when it should go to admin/settings.php?section=modsettingturnitintooltwo

This patch updates the lang files to use the $CFG->wwwroot as a get_string variable.

Cheers, Chris