Tweak to MathJax example

Added a missing comma and used an actual macro so that the MathJax example runs right out of the box upon a copy/paste.

README.md

@@ -1411,9 +1411,9 @@ Reveal.initialize({
 
 	math: {
 		mathjax: 'https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.0/MathJax.js',
-		config: 'TeX-AMS_HTML-full'  // See http://docs.mathjax.org/en/latest/config-files.html
+		config: 'TeX-AMS_HTML-full',  // See http://docs.mathjax.org/en/latest/config-files.html
 		// pass other options into `MathJax.Hub.Config()`
-		TeX: { Macros: macros }
+		TeX: { Macros: { RR: "{\\bf R}" } }
 	},
 
 	dependencies: [