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

Example Input: Sentence: 'a baby pokes his head out of a suitcase'. Replace the letter 'u' with 's' in the sentence.
Example Output: a baby pokes his head ost of a ssitcase

Example Input: Sentence: 'a baseball player is throwing a ball across the dirt field'. Replace the letter 'a' with 't' in the sentence.
Example Output: t btsebtll pltyer is throwing t btll tcross the dirt field

Example Input: Sentence: 'a woman on skiis, skiing down a side of a mountain'. Replace the letter 'd' with 'f' in the sentence.
Example Output:
a woman on skiis, skiing fown a sife of a mountain