Apply suggestions from code review

Co-authored-by: Donal McBreen <dmcbreen@gmail.com>

lib/kamal/cli/server.rb

@@ -1,12 +1,12 @@
 class Kamal::Cli::Server < Kamal::Cli::Base
-  desc "exec", "Run a custom command on the server(use --help to show options)"
+  desc "exec", "Run a custom command on the server (use --help to show options)"
   option :interactive, type: :boolean, aliases: "-i", default: false, desc: "Run the command interactively(use for console/bash)"
   def exec(cmd)
     hosts = KAMAL.hosts | KAMAL.accessory_hosts
 
     case
     when options[:interactive]
-      host = hosts.first
+      host = KAMAL.primary_host
 
       say "Running '#{cmd}' on #{host} interactively...", :magenta