Rename plot xlabel

Histogram was created for birthwgt
Revert "Rename plot xlabel"

This reverts commit 72d17435a1db01a82e3fd308be282e641e0e770e.
Rename plot xlabel

Use full name of variable instead of abbreviation as in previous commit.
Format axis

Axis adjustment is described below but not shown in the code snippet.
Description of axis only describes formatting the x-axis and not the y.

Took the axis values from first.py.
Fix description of parity in assignment

Copied the description from the ch2 solution ipynb.