[ar1_bayes] Review lecture for style-guide compliance

[mmcky]

We need to review ar1_bayes for style-guide compliance and suggestions.

Can you please:

1. checkout the existing PR on branch update_ar1_bayes as the base and build on those changes in a new branch.
2. review the lecture lectures/ar1_bayes for style guide compliance

Here is a copy of the style-guide

quantecon_review_instructions.md