In this task, you need to replace a letter in the sentence with another given letter.

Example Input: Sentence: 'a couple of horses with carriages at a building'. Replace the letter 'u' with 'm' in the sentence.
Example Output: a comple of horses with carriages at a bmilding

Example Input: Sentence: 'woman pushing a cart of luggage in a transportation terminal'. Replace the letter 'a' with 'w' in the sentence.
Example Output: womwn pushing w cwrt of luggwge in w trwnsportwtion terminwl

Example Input: Sentence: 'a police car blocking the street in front of a doubledecker bus'. Replace the letter 'p' with 'a' in the sentence.
Example Output:
a aolice car blocking the street in front of a doubledecker bus