M #-: fix typo (#1799)

(cherry picked from commit 81c288ab1e685fc179abb21847c78f285544efdc)

src/vmm_mad/remotes/lib/vcenter_driver/virtual_machine.rb

@@ -2438,7 +2438,7 @@ end
             if snapshots? || one_snapshots?
                 error_msg =  'Existing sytem snapshots, cannot change disks. '
                 error_msg << 'Please remove all snapshots and try again.'
-                raise error_message
+                raise error_msg
             end
 
             spec_hash     = {}