bugfix: openresty-debug.rb: use 'service' instead of deprecated 'plist'.

[Omoeba]

Have you:

• Followed the guidelines in our Contributing document?
• Checked to ensure there aren't other open Pull Requests for the same update/change?
• Run brew audit --strict --online <formula> (where <formula> is the name of the formula you're submitting) and corrected all errors?
• Built your formula locally prior to submission with brew install <formula>?

New formula: have you

• Written a sensible test? (The best test is to compile and run a sample program.)

Updates to existing formula: have you

• Removed the revision line, if any, when bumping the version number?
• Added/bumped the revision number if the changes affect the precompiled bottles?
• Not altered the bottle section?
• Checked that the tests still pass?

[Omoeba]

This is the same as #33 but fixes openresty-debug.rb instead. It fixes the following warning when brew uses is run.

Warning: Calling plist_options is deprecated! Use service.require_root instead.
Please report this issue to the openresty/brew tap (not Homebrew/brew or Homebrew/homebrew-core), or even better, submit a PR to fix it:
  /opt/homebrew/Library/Taps/openresty/homebrew-brew/Formula/openresty-debug.rb:80