Fix get_experiment_result dialyzer error

[mkurkov]

Overview

Dialyzer returns error about get_experiment_result function type:

Total errors: 1, Skipped: 0, Unnecessary Skips: 0
done in 0m1.18s
lib/growth_book.ex:86:invalid_contract
The @spec for the function does not match the success typing of the function.

Function:
GrowthBook.get_experiment_result/6

Success typing:
@spec get_experiment_result(
  %GrowthBook.Context{:attributes => nil | maybe_improper_list() | map(), _ => _},
  %GrowthBook.Experiment{:hash_attribute => _, :meta => _, :variations => _, _ => _},
  _,
  _,
  _,
  _
) :: %GrowthBook.ExperimentResult{
  :bucket => _,
  :feature_id => _,
  :hash_attribute => _,
  :hash_used? => _,
  :hash_value => _,
  :in_experiment? => boolean(),
  :key => _,
  :name => _,
  :passthrough? => _,
  :sticky_bucket_used? => nil,
  :value => _,
  :variation_id => integer()
}

________________________________________________________________________________
done (warnings were emitted)
Halting VM with exit status 2