dashboard/app: fix comment reference to ExternalConfig struct #18435

Job	Run time
old	6m 59s
dashboard	2m 6s
race	9m 10s
race_dashboard	2m 52s
aux	4m 49s
arch (presubmit_arch_netbsd)	4m 32s
gvisor	4m 2s
arch (presubmit_arch_freebsd)	6m 21s
build	5m 42s
arch (presubmit_arch_linux)	10m 41s
arch (presubmit_arch_executor)	3m 16s
arch (presubmit_arch_darwin)	3m 25s
arch (presubmit_arch_windows)	3m 36s
arch (presubmit_arch_openbsd)	3m 27s
	1h 10m 58s

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