Update template_attributes.js

Correct wrong description of hovertemplate regarding the use of tag `<extra>`

Author: Lexachoc

src/plots/template_attributes.js

@@ -75,7 +75,7 @@ exports.hovertemplateAttrs = function(opts, extra) {
             'The variables available in `hovertemplate` are the ones emitted as event data described at this link https://plotly.com/javascript/plotlyjs-events/#event-data.',
             'Additionally, every attributes that can be specified per-point (the ones that are `arrayOk: true`) are available.',
             descPart,
-            'Anything contained in tag `<extra>` is displayed in the secondary box, for example "<extra>{fullData.name}</extra>".',
+            'Anything contained in tag `<extra>` is displayed in the secondary box, for example `<extra>%{fullData.name}</extra>`.',
             'To hide the secondary box completely, use an empty tag `<extra></extra>`.'
         ].join(' ')
     };