[Book 3] layout error in chapter3.6, equations with `random_double`

[lobneroO]

Just a simple layout error, but in chapter 3.6 the random_double in equations currently gets rendered with a very small d and no underscore. I checked with v3, it was correct in there, so this is a regression for v4.

In TheRestofYourLife in lines 1084 and 1101 it probably should be $$ ... random\textunderscore double ... $$ instead of $$ ... random_double ... $$.

I'm willing to make a pull request but wanted to follow the contribution guidelines

[hollasch]

Fixed in dev branch, commit 48caa61, PR #1330.