Fix backticks in README. #64

Job	Run time
windows (lua53, lua5.3, 5.3, /mingw64/include/lua5.3/)	3m 2s
unix (5.1, ubuntu-latest)	57s
windows (lua, lua, 5.4, /mingw64/include/)	2m 59s
unix (5.1, macos-latest)	1m 58s
windows (luajit, luajit, 5.1, /mingw64/include/luajit-2.1/)	2m 54s
unix (5.2, ubuntu-latest)	57s
unix (5.2, macos-latest)	2m 29s
unix (5.3, ubuntu-latest)	42s
unix (5.3, macos-latest)	1m 55s
unix (5.4, ubuntu-latest)	1m 46s
unix (5.4, macos-latest)	2m 5s
unix (luajit-2.1.0-beta3, ubuntu-latest)	38s
unix (luajit-2.1.0-beta3, macos-latest)	2m 19s
unix (luajit-openresty, ubuntu-latest)	47s
unix (luajit-openresty, macos-latest)	2m 5s
	27m 33s

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