What is the ruling on reciting "وَلَقَدْ تَرَكْنَاها ءَايَةً فَهَلْ مِنْ مُدَّكِرٍ" by substituting the letter 'zay' for the letter 'ra' (in مُدَّكِرٍ)?

If the word in your copy of the Qur’an is written as you mentioned, then this is a typographical error that should not be read. If the letter has a clear dot on it, then it is an ugly distortion of the Qur’an. You must advise the entity that printed this Qur’an and warn people against acquiring this edition. It is better for you to acquire a reliable copy of the Qur’an (such as the Madinah Mushaf), and write on the cover of your copy that contains the error to alert others, and correct the error in the margin. If it is possible to remove the dot without damaging the paper, then do so.