define metricsgrahics-legend class via replacement rather than appending a suffix

[jjallaire]

An upcoming change to htmlwidgets adds and "html-widget" class to all widget html containers. Previously the metricsgrahpics-legend class was establish by appending "-legend" to the current class however this resulted in a class of:

metricsgraphics html-widget-legend

This change modifies the class via string replacement, yielding:

metricsgraphics-legent html-widget

[hrbrmstr]

ty