Use object_cast instead of assign

Developers_manual/doc_tex/Developers_manual/objects.tex

@@ -66,7 +66,7 @@ The following piece of code is taken from the above \cgal\ intersection routine:
 There is only one member operation defined for \ccc{Object}, a test for
 an empty object. In order to make use of the returned object, 
 you can try to assign it to the possible return types using 
-\ccc{CGAL::assign(Candidate_return_type, Object)}\ccIndexGlobalFunction{assign}
+\ccc{CGAL::object_cast}
 function; see also the definition of the class 
 \ccAnchor{http://www.cgal.org/Manual/doc_html/kernel/Class_Object.html}{\ccc{Object}} in the 
 \cgal\ kernel manual.