org-manual.org: Update links to MathJax docs

* doc/org-manual.org (Footnotes): Fix links to particular sections in
MathJax manual.

Scheme is not changed to https: since the site prefers http:

    curl -I https://docs.mathjax.org/
    HTTP/2 302
    date: Sun, 03 Oct 2021 16:04:15 GMT
    location: http://docs.mathjax.org/en/latest/

doc/org-manual.org

@@ -22479,9 +22479,9 @@ semantic relevance.
 
 [fn:130] Please note that exported formulas are part of an HTML
 document, and that signs such as =<=, =>=, or =&= have special
-meanings.  See [[http://docs.mathjax.org/en/latest/tex.html#tex-and-latex-in-html-documents][MathJax TeX and LaTeX support]].
+meanings.  See [[http://docs.mathjax.org/en/latest/input/tex/html.html#tex-and-latex-in-html-documents][MathJax TeX and LaTeX in HTML documents]].
 
-[fn:131] See [[http://docs.mathjax.org/en/latest/tex.html#tex-extensions][TeX and LaTeX extensions]] in the [[http://docs.mathjax.org][MathJax manual]] to learn
+[fn:131] See [[http://docs.mathjax.org/en/latest/input/tex/extensions.html#tex-and-latex-extensions][TeX and LaTeX extensions]] in the [[http://docs.mathjax.org][MathJax manual]] to learn
 about extensions.
 
 [fn:132] If the classes on TODO keywords and tags lead to conflicts,