Set encryption=on

encryption=aes-256-gcm is now the default.

anarcat mentioned this in PR #306.

Signed-off-by: Richard Laager <rlaager@wiktel.com>

docs/Getting Started/NixOS/Root on ZFS/2-system-configuration.rst

@@ -172,7 +172,7 @@ System Configuration
       zfs create \
        -o canmount=off \
        -o mountpoint=none \
-       -o encryption=aes-256-gcm \
+       -o encryption=on \
        -o keylocation=prompt \
        -o keyformat=passphrase \
        rpool_$INST_UUID/$INST_ID