Update font_attributes.js

Author: LiamConnors

src/plots/font_attributes.js

@@ -55,7 +55,7 @@ module.exports = function(opts) {
                 'The Chart Studio Cloud (at https://chart-studio.plotly.com or on-premise) generates images on a server,',
                 'where only a select number of',
                 'fonts are installed and supported.',
-                'These include *Arial*, *Balto*, *Courier New*, *Droid Sans*,, *Droid Serif*,',
+                'These include *Arial*, *Balto*, *Courier New*, *Droid Sans*, *Droid Serif*,',
                 '*Droid Sans Mono*, *Gravitas One*, *Old Standard TT*, *Open Sans*, *Overpass*,',
                 '*PT Sans Narrow*, *Raleway*, *Times New Roman*.'
             ].join(' ')