docs: userguide: labels-custom-metadsata.md: update syntax for emtpy value labels
docs: userguide: labels-custom-metadsata.md: update syntax for emtpy value labels

Signed-off-by: Antonio Murdaca <f9809039ea4850e75dcb8d80b778844aa463358f@redhat.com>

Zebrilee/cli,ashfall/cli,docker/cli,e11137/cli,dnephin/cli,thaJeztah/cli,ashfall/cli,redpanda/cli,e11137/cli,cpuguy83/cli,cpuguy83/cli,allencloud/cli,Zebrilee/cli,riyazdf/cli,riyazdf/cli,allencloud/cli,thaJeztah/cli,docker/cli,dnephin/cli,redpanda/cli