quincy: doc/dev: format command in cephfs-mirroring

[zdover23]

Correctly format a command in doc/dev/cephfs-mirroring/#creating-users.

Reported by casanlin@init7.net at
https://pad.ceph.com/p/Report_Documentation_Bugs

Signed-off-by: Zac Dover zac.dover@proton.me
(cherry picked from commit 408219b)

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