A few minor changes to content-mathml

These are mostly cosmetic changes:

*  Fix the comment to describe the content-mathml code rather than the file you copied from.  :-)

*  Call `MATHML.Parse.prototype` directly rather than make a special flag for preprocessing in the `MATHML.Parse` initialization routine.

*  Use `MATHML` variable rather than full reference to `MathJax.InputJax.MathML`.