chore(readme): fix label description (#19) #34

Job	Run time
test (ubuntu-20.04, 5.1)	17s
test (ubuntu-20.04, 5.2)	18s
test (ubuntu-20.04, 5.3)	18s
test (ubuntu-20.04, 5.4)	23s
test (ubuntu-20.04, luajit)	27s
test (ubuntu-20.04, luajit-openresty)	17s
test (macos-11.0, 5.1)	53s
test (macos-11.0, 5.2)	51s
test (macos-11.0, 5.3)	52s
test (macos-11.0, 5.4)	55s
test (macos-11.0, luajit)	51s
test (macos-11.0, luajit-openresty)	48s
test (macos-latest, 5.4)	55s
test (windows-latest, 5.1, msvc)	1m 16s
test (windows-latest, 5.2, msvc)	1m 14s
test (windows-latest, 5.3, msvc)	1m 12s
test (windows-latest, 5.4, msvc)	1m 2s
test (windows-latest, luajit, mingw)	1m 0s
	13m 49s

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