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

[President Camacho]

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.png

should be updated to reflect current level settings as well.