fix label in generated form for relations
the label for object relations was incorrect, because the '_object' mangling is
done on the wrong component of field.label.
fix ticket 472831
Document: cubicweb-doc
Title: CubicWeb documentation
Author: Logilab
Abstract: Some base documentation for CubicWeb users and developpers
Section: Apps/Programming
Format: HTML
Index: /usr/share/doc/cubicweb-documentation/index.html
Files: /usr/share/doc/cubicweb-documentation/*.html