doc(examples): replace nested_service by fallback_service (#438) #1131

Job	Run time
msrv	23s
format	11s
test	1m 43s
udeps	52s
examples	28s
feature_set	4m 51s
rust-clippy-analyze	48s
engine_io (v3)	1m 15s
socket_io (v4)	50s
engine_io (v4)	35s
socket_io (v5)	55s
socket_io (v5-msgpack)	57s
adapter (v4, fred-e2e)	2m 11s
adapter (v4, redis-e2e)	1m 10s
adapter (v4, redis-cluster-e2e)	1m 37s
adapter (v4, fred-cluster-e2e)	1m 29s
adapter (v4-msgpack, fred-e2e)	1m 14s
adapter (v4-msgpack, redis-e2e)	1m 15s
adapter (v4-msgpack, redis-cluster-e2e)	1m 13s
adapter (v4-msgpack, fred-cluster-e2e)	1m 10s
adapter (v5, fred-e2e)	1m 11s
adapter (v5, redis-e2e)	1m 12s
adapter (v5, redis-cluster-e2e)	1m 37s
adapter (v5, fred-cluster-e2e)	1m 10s
adapter (v5-msgpack, fred-e2e)	1m 9s
adapter (v5-msgpack, redis-e2e)	1m 11s
adapter (v5-msgpack, redis-cluster-e2e)	1m 36s
adapter (v5-msgpack, fred-cluster-e2e)	1m 15s
all_passed	1s
	35m 29s

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