fix: Add fare_media_id to fare_products in spec.R

[rymarczy]

fare_products SPEC is missing fare_media_id as described in GTFS Specification.

https://gtfs.org/schedule/reference/#fare_productstxt

[codecov-commenter]

Codecov Report

Merging #207 (6b47aa7) into master (9e8b787) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 6b47aa7 differs from pull request most recent head 26f3f17. Consider uploading reports for the commit 26f3f17 to get more accurate results

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@@            Coverage Diff            @@
##            master      #207   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           16        16           
  Lines         1212      1212           
=========================================
  Hits          1212      1212           

Impacted Files	Coverage Δ
R/spec.R	100.00% <100.00%> (ø)

[polettif]

Thanks!