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

Example Input: Sentence: 'young boy leaping into air with tennis racket with dog below him'. Replace the letter 't' with 'r' in the sentence.
Example Output: young boy leaping inro air wirh rennis racker wirh dog below him

Example Input: Sentence: 'a person with a cell phone standing next to a train'. Replace the letter 's' with 't' in the sentence.
Example Output: a perton with a cell phone ttanding next to a train

Example Input: Sentence: 'a group of people are looking at a sleeping bear'. Replace the letter 'i' with 'v' in the sentence.
Example Output:
a group of people are lookvng at a sleepvng bear