Merge pull request #3 from rknell/patch-2

Update domains.rb

Author: SebastianSzturo

lib/dokku_cli/domains.rb

@@ -8,7 +8,7 @@ def domains
 
     desc "domains:add DOMAIN", "Add a custom domain to the app"
     def domains_add(custom_domain)
-      run_command "domains:set #{app_name} #{custom_domain}"
+      run_command "domains:add #{app_name} #{custom_domain}"
     end
 
     desc "domains:clear","Clear all custom domains for app"