chore(gatsby-remark-katex): Change to new URL and avoid redirect

[acagastya]

Description

Updated katex repo URL to current one, that is (https://github.com/KaTeX/KaTeX).
Previously, it was (https://github.com/Khan/KaTeX) and it is now a redirect.

[LekoArts]

Thanks 👍

[gatsbot]

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