Allow clippy::ref_option lint for serde::format_description.

[wojciech-graj]

time::serde::format_description! generates a serialize function that takes &Option<T>, which is picked up by the clippy::ref_option lint for an end user.

error: it is more idiomatic to use `Option<&T>` instead of `&Option<T>`
  |
  | time::serde::format_description!(iso8601, Date, "[year]-[month]-[day]");
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ref_option

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.4%. Comparing base (ec327a2) to head (5800572).
Report is 5 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #732   +/-   ##
=====================================
  Coverage   93.4%   93.4%           
=====================================
  Files         89      89           
  Lines      10128   10129    +1     
=====================================
+ Hits        9458    9461    +3     
+ Misses       670     668    -2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.