Update docstring

Co-authored-by: Alex Johnson <alex@plot.ly>

Author: emilykl

src/traces/bar/layout_attributes.js

@@ -57,7 +57,7 @@ module.exports = {
         editType: 'calc',
         description: [
             'Sets the rounding of bar corners. May be an integer number of pixels,',
-            'or a percentage of bar width(as a string).'
+            'or a percentage of bar width (as a string ending in %).'
         ].join(' ')
     },
 };