Fix invalid math formulas in quantization doc

A single backslash is not properly escaped in the web documentation. We can make sure to escape for rendering subscripts.

Additionally, it also fixed the mal-formed equations in //"Affine to fixed point"// and //"Fixed point to affine"// sections. With this fix, the page is rendered as follows.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D101252