[nexus] [trivial] Fix doc comments on `RouteDestination` #37841

Job	Run time
check-features	10m 44s
check-style	20s
check-omicron-deployment (ubuntu-22.04)	8m 39s
clippy-lint	10m 27s
check-omicron-deployment (macos-14)	10m 3s
build-docs	8m 40s
	48m 53s

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