mathematical notation bugs in docs

Dear Tobias & crew,

I'm very glad you have mathematical descriptions of the transforms - this is essential for a professional math library.
But there are a bunch of typos in
https://github.com/JuliaMath/NFFT.jl/blob/master/docs/src/background.md
Mostly this is bold vs italic,
but also [-1/2,1/2) etc.

You may want to explain that there are other definitions,
eg
https://finufft.readthedocs.io/en/latest/math.html
and to tell the user to rescale their NU points to change definition.

I'll try to do a PR but may not be able to get to it :)

Keep up the good work. Best, Alex