Fix route type name (unavailable -> unreachable)

Closed Etienne Champetier requested to merge champtar/NetworkManager:route-unreachable into main

Fixes: 1cc3d00c ('libnm/doc: list route attributes in man nm-settings-nmcli')

Fixes: 76634987 ('platform/trivial: add code comments for NMPGlobalTracker')

Edited by Etienne Champetier

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