Small edits to performance section of the doc #1824

Job	Run time
VMD / test_backends	53s
GROMACS (release-2023) / test_backends	1m 51s
GROMACS (main) / test_backends	2m 23s
GROMACS (release-2022) / test_backends	2m 5s
LAMMPS (develop) / test_backends	10m 15s
NAMD (devel) / test_backends	5m 20s
	22m 47s

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