haproxy_ctld_daemon.rb: Fix backtrace option

The haproxy_ctld_daemon.rb script in the haproxy cartridge had a typo:
':backgrace' instead of ':backtrace'.

This typo appears to have been present since haproxy_ctld_daemon.rb was
introduced in commit ee8cebbe1adb351a3b80ca3fda4f047aee8fac1b.