[2.0.x] Fixed outdated comment in RelatedObjectDoesNotExist.

Backport of 0a69479b6c from master

django/db/models/fields/related_descriptors.py

@@ -88,8 +88,8 @@ class ForwardManyToOneDescriptor:
     @cached_property
     def RelatedObjectDoesNotExist(self):
         # The exception can't be created at initialization time since the
-        # related model might not be resolved yet; `rel.model` might still be
-        # a string model reference.
+        # related model might not be resolved yet; `self.field.model` might
+        # still be a string model reference.
         return type(
             'RelatedObjectDoesNotExist',
             (self.field.remote_field.model.DoesNotExist, AttributeError),