Standardize on bmatrix (instead of pmatrix) for LaTeX Math in Lectures

[mmcky]

Some lectures have deviated using pmatrix instead of bmatrix: https://manual.quantecon.org/writing/styleguide.html

Such ashttps://python.quantecon.org/finite_markov.html and probably elsewhere.

It would be great to regex across the lecture for pmatrix and update