Add multi-argument example to latexstring docs

[stillyslalom]

ref. https://discourse.julialang.org/t/combining-latex-with-variables-in-pyplot-title/41128/3?u=stillyslalom

[stevengj]

Thanks!

[stevengj]

Note that this is not a great example, however, since it produces L"an equation: $1 + \alpha^2 = $3.142" — that is, the 3.142 is outside the LaTeX equation, which may not be formatted the way you want.