Update start.rst
Update start.rst

Fixed https address of CDN.
Axure/MathJax-docs,mitya57/mathjax-docs,GerHobbelt/MathJax-docs,Axure/MathJax-docs,mathjax/MathJax-docs,GerHobbelt/MathJax-docs,mathjax/MathJax-docs,mitya57/mathjax-docs