docs: update Markdown equation elements

[stdlib-bot]

This PR

• updates equation elements found in Markdown files

[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
math/base/special/cothf	$\\color{green}170/170$ $\\color{green}+100.00\\%$	$\\color{green}5/5$ $\\color{green}+100.00\\%$	$\\color{green}2/2$ $\\color{green}+100.00\\%$	$\\color{green}170/170$ $\\color{green}+100.00\\%$
stats/incr/nancv	$\\color{green}141/141$ $\\color{green}+100.00\\%$	$\\color{green}11/11$ $\\color{green}+100.00\\%$	$\\color{green}2/2$ $\\color{green}+100.00\\%$	$\\color{green}141/141$ $\\color{green}+100.00\\%$
stats/incr/nanmstdev	$\\color{green}230/230$ $\\color{green}+100.00\\%$	$\\color{green}11/11$ $\\color{green}+100.00\\%$	$\\color{green}2/2$ $\\color{green}+100.00\\%$	$\\color{green}230/230$ $\\color{green}+100.00\\%$

The above coverage report was generated for the changes in this PR.