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Say "first iteration" in the 1st itr legend #2120

Job	Run time
benchmark-run-once	1m 29s
test (ruby)	51s
benchmark-default (ruby)	12m 57s
test (truffleruby)	1m 40s
benchmark-ractor-only (head)	54s
benchmark-graph	39s
benchmark-default (head)	13m 20s
benchmark-ractor (head)	18m 28s
benchmark-default (truffleruby, --excludes=knucleotide,lobsters,ruby-lsp,shipit)	7m 34s
test (head)	40s
all-tests	3s
	58m 35s