add a description to the update Rake task

adding a description makes the task being listed when running `rake -T`

Rakefile

@@ -42,6 +42,7 @@ task :install_prezto do
   end
 end
 
+desc 'Updates the installation'
 task :update do
   Rake::Task["vundle_migration"].execute if needs_migration_to_vundle?
   Rake::Task["install"].execute