doc: Fix bad snapshot syntax #1332

Job	Run time
Code	7m 4s
Documentation	6m 11s
Client (1.21.x, ubuntu-latest)	1m 8s
Client (1.21.x, macos-latest)	1m 11s
Client (1.21.x, windows-latest)	1m 49s
System (1.21.x, cluster, dir)	12m 2s
System (1.21.x, cluster, btrfs)	12m 14s
System (1.21.x, cluster, lvm)	13m 18s
System (1.21.x, cluster, zfs)	12m 55s
System (1.21.x, cluster, ceph)	15m 39s
System (1.21.x, cluster, random)	12m 7s
System (1.21.x, standalone, dir)	21m 10s
System (1.21.x, standalone, btrfs)	20m 35s
System (1.21.x, standalone, lvm)	39m 35s
System (1.21.x, standalone, zfs)	39m 13s
System (1.21.x, standalone, ceph)	1h 13m 15s
System (1.21.x, standalone, random)	33m 49s
System (stable, cluster, dir)	10m 44s
System (stable, standalone, dir)	21m 31s
System (tip, cluster, dir)	11m 51s
System (tip, standalone, dir)	21m 13s
	6h 28m 34s

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