[wealth_dynamics] Update lecture to comply with QuantEcon style guide (titles)

[mmcky]

I would like to review lectures/wealth_dynamics for compliance with the quantecon style guide

Please review the following rules

1. Capitalize ALL words in lecture titles.

Correct: "How it Works: Data, Variables and Names"
Incorrect: "How it works: data, variables and names"

2. Capitalize ONLY the first word and proper nouns in all other headings (sections, subsections, etc.).

Correct: "Binary packages with Python frontends"
Incorrect: "Binary Packages with Python Frontends"

and update the lecture.