docs(example_performance.md): correct a sub-heading (#896) #830

Job	Run time
linux-64-py3.10	6m 38s
macOS	3m 25s
estimation_mode_test	17m 0s
win-64	2m 39s
prototype_mtc	50m 5s
develop-docbuild	4m 2s
placeholder_psrc	43m 34s
Standard 1-Zone Example (MTC)	53m 9s
prototype_marin	2m 4s
random_seed_generation_test	3m 8s
Standard 2-Zone Example (SANDAG)	1h 9m 52s
prototype_mtc_extended	1h 26m 59s
placeholder_sandag	1h 45m 26s
prototype_sandag_xborder	5m 12s
production_semcog	5m 51s
prototype_mwcog	52m 8s
placeholder_multiple_zone	8m 47s
	8h 39m 59s

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