Update start.rst
Update start.rst

Fixed forgotten 2.0 references for 2.1 release
GerHobbelt/MathJax-docs,mitya57/mathjax-docs,mathjax/MathJax-docs,mitya57/mathjax-docs,Axure/MathJax-docs,GerHobbelt/MathJax-docs,Axure/MathJax-docs,mathjax/MathJax-docs