Remove extra newline at the end of an HS descriptor

In rend_encode_v2_descriptors, an extra newline is added at the end of the descriptor. However, rend-spec.txt doesn't specify this extra newline.

atagar mentionned on IRC that it would be surprising if that is the intended behaviour since other type of descriptors don't have it:

< atagar> It's the only descriptor type with it, so I'd be surprised if it was intended. ;)

Thus this should be fixed in the tor code and not in the spec.