doc: fix rbd cfg string
The cfg string for rbd should not have a initial "/".
@@ -72,7 +72,7 @@ o- backstores .......................................................... [...]
/backstores> cd user:rbd
-/backstores/user:rbd> create cfgstring=/pool/rbd1 name=rbd0 size=1G
+/backstores/user:rbd> create cfgstring=pool/rbd1 name=rbd0 size=1G
Created user-backed storage object rbd0 size 1073741824.
