Add assert_sum to example file

[czepluch]

The assert_sum is currently not in the example file. We should add it there to have an up to date representation of the functionality of the SP.

[christianbrb]

@czepluch

It is here in L91

scenario-player/examples/scenario-example-v2.yaml

Lines 77 to 92 in 48d2d74

	## Available task types:
	## - serial
	## - parallel
	## - open_channel
	## - close_channel
	## - deposit
	## - withdraw
	## - transfer
	## - stop_node
	## - kill_node
	## - start_node
	## - update_node_options
	## - assert
	## - assert_all
	## - assert_sum
	## - wait

Does it need more explanation?