Doc: Clusters from Scratch: Fix firewall-cmd --add-rich-rule

The firewall-cmd --add-rich-rule command in shared-storage.rst doesn't
like spaces at the beginning of the second line.

[root@pcmk-1 ~]# firewall-cmd --permanent --add-rich-rule='rule family="ipv4" \

source address="192.168.122.102" port port="7789" protocol="tcp" accept'

Error: INVALID_RULE: unknown element

It works fine if we don't start the second line with spaces.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>