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
stats/base/dists/invgamma/cdf	$\\color{green}299/299$ $\\color{green}+100.00\\%$	$\\color{green}25/25$ $\\color{green}+100.00\\%$	$\\color{green}4/4$ $\\color{green}+100.00\\%$	$\\color{green}299/299$ $\\color{green}+100.00\\%$
stats/base/dists/invgamma/entropy	$\\color{green}201/201$ $\\color{green}+100.00\\%$	$\\color{green}10/10$ $\\color{green}+100.00\\%$	$\\color{green}2/2$ $\\color{green}+100.00\\%$	$\\color{green}201/201$ $\\color{green}+100.00\\%$
stats/base/dists/invgamma/kurtosis	$\\color{green}203/203$ $\\color{green}+100.00\\%$	$\\color{green}10/10$ $\\color{green}+100.00\\%$	$\\color{green}2/2$ $\\color{green}+100.00\\%$	$\\color{green}203/203$ $\\color{green}+100.00\\%$
stats/base/dists/invgamma/logpdf	$\\color{green}313/313$ $\\color{green}+100.00\\%$	$\\color{green}25/25$ $\\color{green}+100.00\\%$	$\\color{green}4/4$ $\\color{green}+100.00\\%$	$\\color{green}313/313$ $\\color{green}+100.00\\%$
stats/base/dists/invgamma/mean	$\\color{green}192/192$ $\\color{green}+100.00\\%$	$\\color{green}10/10$ $\\color{green}+100.00\\%$	$\\color{green}2/2$ $\\color{green}+100.00\\%$	$\\color{green}192/192$ $\\color{green}+100.00\\%$
stats/base/dists/invgamma/mode	$\\color{green}195/195$ $\\color{green}+100.00\\%$	$\\color{green}10/10$ $\\color{green}+100.00\\%$	$\\color{green}2/2$ $\\color{green}+100.00\\%$	$\\color{green}195/195$ $\\color{green}+100.00\\%$
stats/base/dists/invgamma/pdf	$\\color{green}313/313$ $\\color{green}+100.00\\%$	$\\color{green}25/25$ $\\color{green}+100.00\\%$	$\\color{green}4/4$ $\\color{green}+100.00\\%$	$\\color{green}313/313$ $\\color{green}+100.00\\%$
stats/base/dists/invgamma/skewness	$\\color{green}193/193$ $\\color{green}+100.00\\%$	$\\color{green}10/10$ $\\color{green}+100.00\\%$	$\\color{green}2/2$ $\\color{green}+100.00\\%$	$\\color{green}193/193$ $\\color{green}+100.00\\%$
stats/base/dists/invgamma/stdev	$\\color{green}196/196$ $\\color{green}+100.00\\%$	$\\color{green}10/10$ $\\color{green}+100.00\\%$	$\\color{green}2/2$ $\\color{green}+100.00\\%$	$\\color{green}196/196$ $\\color{green}+100.00\\%$
stats/base/dists/invgamma/variance	$\\color{green}196/196$ $\\color{green}+100.00\\%$	$\\color{green}10/10$ $\\color{green}+100.00\\%$	$\\color{green}2/2$ $\\color{green}+100.00\\%$	$\\color{green}196/196$ $\\color{green}+100.00\\%$

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

[kgryte]

I do not believe these changes are warranted.