Update description of attribute source in image traces

Author: antoinerg

src/traces/image/attributes.js

@@ -29,7 +29,7 @@ module.exports = extendFlat({
         editType: 'calc',
         description: [
             'Specifies the data URI of the image to be visualized.',
-            'The URI consists of "data:[<media type>][;base64],<data>"'
+            'The URI consists of "data:image/[<media subtype>][;base64],<data>"'
         ].join(' ')
     },
     z: {