book: fix suggested configuration syntax to merge rather than override#1959

sholderbach merged 2 commits intonushell:mainnushell/nushell.github.io:mainfrom

Axlefublr:Axlefublr/coloring_and_themingAxlefublr/nushell.github.io:Axlefublr/coloring_and_themingCopy head branch name to clipboard