[Attributor][NFC] Update description for the dependency graph

The word "dependency graph" is a bit misleading. When there is an
edge from node A to B (A -> B), it actually mean that B depends on
A and when the state of A is updated, B should also be updated. So
I update the comment to make the description clearer.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D85065