[Tor www-team] tb-manual/security-slider needs an update

https://tb-manual.torproject.org/en-US/security-slider.html An update is needed for URL above. "Security Levels" information should be updated to replace "High, Medium, Low" levels with the current "Safest, Safer, Standard" information/configuration. * The screen-shot of: https://tb-manual.torproject.org/en-US/media/security-slider/slider_window.p... should be updated to reflect current level settings as well.