minor fix
minor fix

fixing link to MathML spec
mathjax/MathJax-docs,mitya57/mathjax-docs,GerHobbelt/MathJax-docs,mathjax/MathJax-docs,GerHobbelt/MathJax-docs,mitya57/mathjax-docs,Axure/MathJax-docs,Axure/MathJax-docs