Update Vagrantfile

correct wrong grammar
can running => can be running

Author: jasl

Vagrantfile

@@ -14,7 +14,7 @@ Vagrant.configure('2') do |config|
     override.vm.box = 'parallels/ubuntu-12.04'
     override.vm.box_url = 'https://vagrantcloud.com/parallels/ubuntu-12.04'
 
-    # Can running at background, see https://github.com/Parallels/vagrant-parallels/issues/39
+    # Can be running at background, see https://github.com/Parallels/vagrant-parallels/issues/39
     v.customize ['set', :id, '--on-window-close', 'keep-running']
   end