fix typo in mergerfs readme

roles/mergerfs/README.md

@@ -6,7 +6,7 @@ Installs mergerfs and mergerfs tools. Creates mountpoints and configures /etc/fs
 Variables
 ---------
 
-    mergerfs_disk:
+    mergerfs_disks:
       - path: /mnt/disk1
         disk: /dev/disk/by-id/ata-WDC_WD101EFBX-68B0AN0_VCJW0TDP
         owner: username # defaults to root