fix attribute completeness description text

[JanReifenrath]

The attribute completeness description text says "attributes" even if there is only a single attribute selected. It should only say "attribute"

[JanReifenrath]

the code should look roughly like this
if len(self.attribute_names) > 1:
tags = "attributes " + ", ".join(self.attribute_names)
else:
tags = "attribute " + self.attribute_names[0]