fix lvcreate command

-l and -L are easy to confuse, just use --size here

howto/lvm.md

@@ -234,7 +234,7 @@ that we have available free space in the existing volume group.
 Create the new LV, we'll call it `srv` since it will be mounted at
 `/srv`:
 
-    lvcreate -n srv -l50G vg_ganeti
+    lvcreate -n srv -L50G vg_ganeti
     mkfs -t ext4 /dev/vg_ganeti/srv
 
 Mount the new volume at a temporary location: