Rearrange braces to work with TeX binaries

Hello Geoff.

I made some changes to settings.php. Mainly removed extraneous braces or swapped the order of braces and dollar signs if braces are needed to make the expressions work with both MathJax and with the TeX binaries.  I also changed the unicode character to the correct ASCII TeX expression.  MathJax processes unicode only because it is looking at HTML documents, but the is really not a standard extension of TeX and causes compatibility problems. An editor should produce correct TeX expressions if possible.  The will enable Moodle documents to be copied to be easily used with other programs which understand TeX.