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

Example Input: Sentence: 'a polar bear relaxing in a man-made pool'. Replace the letter 'r' with 'x' in the sentence.
Example Output: a polax beax xelaxing in a man-made pool

Example Input: Sentence: 'a person is looking out of a large window'. Replace the letter 'o' with 'p' in the sentence.
Example Output: a perspn is lppking put pf a large windpw

Example Input: Sentence: 'a person jumping a skate board in the air'. Replace the letter 'a' with 'k' in the sentence.
Example Output:
k person jumping k skkte bokrd in the kir