Transcribing DNA into RNA
An RNA string is a string formed from the alphabet containing 'A', 'C', 'G', and 'U'.
Given a DNA string t corresponding to a coding strand, its transcribed RNA string u is formed by replacing all occurrences of 'T' in t with 'U' in u.
Given: A DNA string t having length at most 1000 nt.
Return: The transcribed RNA string of t.