docs(example_performance.md): correct a sub-heading #818

Job	Run time
linux-64-py3.10	6m 28s
macOS	6m 16s
prototype_mtc	48m 9s
develop-docbuild	0s
win-64	11m 12s
estimation_mode_test	17m 56s
placeholder_psrc	37m 17s
prototype_marin	2m 56s
random_seed_generation_test	4m 10s
prototype_mtc_extended	1h 17m 38s
Standard 1-Zone Example (MTC)	49m 16s
placeholder_sandag	1h 36m 16s
Standard 2-Zone Example (SANDAG)	1h 0m 23s
prototype_sandag_xborder	5m 34s
production_semcog	6m 32s
prototype_mwcog	45m 12s
placeholder_multiple_zone	9m 16s
	8h 4m 31s

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