[money_inflation_nonlinear] Editorial Suggestions

Suggestions by @longye-tian

Code

[x] Remove the grid.
[x] Change the legend for the Laffer curve plot to "Laffer curve". Also, use default colors in matplotlib.

[x] Bold face is reserved for definitions and italics are used for emphases.
[x] Only the first word in the title should be capitalized.
[x] Migrate some of "This lecture studies" to "We study ...".
[x] Maintain consistent use of bar and overline.
[x] (28.3) repeats (28.2). Consider removing (28.3) and adjusting the links to (28.2) if (28.3) is removed.
[x] Define all the notations like $\pi$ and $m$.
[x] "limiting values exists" -> exist
[x] "...that studies a linear version of the model in this lecture`" -> "that studies a linear version of the model in this lecture."
[ ] Give a theoretical explanation for the patterns in Figure 28.2.
[x] In Equation (28.6) we have $x: x \ge 0$. Remove "x:", and put a curly bracket covering the equation after max to indicate the scope of max.