Fix multiple disks documented example test (rabbitmq#955)

Author: coro

docs/examples/multiple-disks/test.sh

@@ -4,5 +4,5 @@ set -ex
 kubectl exec -t multiple-disks-server-0 -c rabbitmq -- rabbitmqctl environment > rabbitmq-environment.out
 
 grep 'data_dir,"/var/lib/rabbitmq/quorum-segments"' rabbitmq-environment.out
-grep "{wal_data_dir,'/var/lib/rabbitmq/quorum-wal'}" rabbitmq-environment.out
+grep 'wal_data_dir,"/var/lib/rabbitmq/quorum-wal"' rabbitmq-environment.out