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

Example Input: Sentence: 'a bike sitting parked next to a cart in front of some boxes'. Replace the letter 't' with 'x' in the sentence.
Example Output: a bike sixxing parked nexx xo a carx in fronx of some boxes

Example Input: Sentence: 'bags of seeds with a pair of black scissors sitting on them'. Replace the letter 'b' with 'y' in the sentence.
Example Output: yags of seeds with a pair of ylack scissors sitting on them

Example Input: Sentence: 'an old brown train in a busy city'. Replace the letter 'y' with 'l' in the sentence.
Example Output:
an old brown train in a busl citl