Change uSageMath to something else or clarify it

[kcrisman]

One thing I find confusing:

<li><a href="{{ 'help-uSage.html'|prefix }}">u{{ sage }} User Groups</a></li>

Amazingly, the link etc. works, but the u in both instances is really confusing. I guess maybe just "Sage User Groups" would be easier" - I assume the u was discussed on sage-devel at some point, maybe "uS[eS]agemath" like uSeR? But for R it rolls off the tongue easier and makes sense.

Anyway, the current link looks like a typo.

[haraldschilly]

SMUG: sage math user group

:smile:

[kcrisman]

(Actually, another person once started a SLUG - Sage learner user group.)