Fix reference to Trac ticket in doctest

Author: egourgoulhon

src/sage/manifolds/differentiable/diff_form_module.py

@@ -796,7 +796,7 @@ def _element_constructor_(self, comp=[], frame=None, name=None,
             sage: A(0) is A.zero()
             True
 
-        Check that #27658 is fixed::
+        Check that :trac:`27658` is fixed::
 
             sage: f = M.scalar_field(x)
             sage: f in A