docs: update agreement README with committableEvent

[ghost]

the mentioned proposalCommittableEvent does not exist in this form within the source code.

The correct event is committableEvent

go-algorand/agreement/events.go

Lines 494 to 500 in 30acd70

	type committableEvent struct {
	// Proposal is set to be the proposal-value which is committable.
	Proposal proposalValue
	// the proposal-vote that authenticated the payload (if one exists)
	Vote vote
	}

[bbroder-algo]

yes.