[MLIR][Quant] Fix equations in `Quantization.md`

This patch fixes the equations on the Quantization page
(https://mlir.llvm.org/docs/Quantization/).

I don't know what caused the equations to be broken, it
might be https://github.com/llvm/mlir-www/pull/152, but
I'm not sure. Irregardless, let's just fix it and be
done with it.

I've fixed the equations by moving some subscripts to
the text. For some reason, the large number of subscripts
caused Mathjax to fail. I've also tried KaTeX, which
failed at exactly the same number of subscripts.

The workflow to inspect the fix is as follows:

```
$ git clone --depth=1 https://github.com/llvm/mlir-www.git /some/path/mlir-www

$ git clone --depth=1 https://github.com/llvm/llvm-project.git /some/path/llvm-project

$ cp /some/path/llvm-project/mlir/docs/Quantization.md \
    /some/path/mlir-www/website/content/Quantization.md

$ cd /some/path/mlir-www/website

$ hugo serve
[...]
Web Server is available at http://localhost:1313/ (bind address 127.0.0.1)
Press Ctrl+C to stop
```
and view the page at http://localhost:1313/Quantization/.

Reviewed By: stellaraccident

Differential Revision: https://reviews.llvm.org/D152651