Merge pull request #4136 from edx/jdmulloy/fix_hostsupdater

Fix spelling of vagrant-hostsupdater

vagrant/release/fullstack/Vagrantfile

 Vagrant.require_version ">= 1.8.7"
-unless Vagrant.has_plugin?("vagrant-hostupdater")
-  raise "Please install the vagrant-hostupdater plugin by running `vagrant plugin install vagrant-hostupdater`"
+unless Vagrant.has_plugin?("vagrant-hostsupdater")
+  raise "Please install the vagrant-hostsupdater plugin by running `vagrant plugin install vagrant-hostsupdater`"
 end
 
 VAGRANTFILE_API_VERSION = "2"