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

Ex Input:
Sentence: 'a couple walking down a sidewalk next to some graffiti'. Replace the letter 'a' with 'y' in the sentence.

Ex Output:
y couple wylking down y sidewylk next to some gryffiti


Ex Input:
Sentence: 'an old rusty car sits outside of an old building'. Replace the letter 'o' with 'r' in the sentence.

Ex Output:
an rld rusty car sits rutside rf an rld building


Ex Input:
Sentence: 'a red double decker bus coming down the road'. Replace the letter 'c' with 'b' in the sentence.

Ex Output:

a red double debker bus boming down the road