Fix method name in README.md

Looking at line 105 in `theme-set.js`, this should be `newThemeSet`.

This makes me wonder if `Gauge.prototype.setThemeset` should be `Gauge.prototype.setThemeSet` for consistency, but that is outside of the scope of this PR and would be a breaking change.

README.md

@@ -207,7 +207,7 @@ var ThemeSet = require('gauge/theme-set')
 var themes = new ThemeSet()
 // or
 var themes = require('gauge/themes')
-var mythemes = themes.newThemeset() // creates a new themeset based on the default themes
+var mythemes = themes.newThemeSet() // creates a new themeset based on the default themes
 ```
 
 #### themes(opts)