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

Example Input: Sentence: 'a small train is on the tracks by a house'. Replace the letter 'l' with 'x' in the sentence.
Example Output: a smaxx train is on the tracks by a house

Example Input: Sentence: 'a boy on the beach playing with a frisbee'. Replace the letter 'h' with 'e' in the sentence.
Example Output: a boy on tee beace playing wite a frisbee

Example Input: Sentence: 'a girl riding skis holding two ski poles'. Replace the letter 'k' with 'w' in the sentence.
Example Output:
a girl riding swis holding two swi poles