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

Example Input: Sentence: 'a couple walking down a sidewalk next to some graffiti'. Replace the letter 'a' with 'y' in the sentence.
Example Output: y couple wylking down y sidewylk next to some gryffiti

Example Input: Sentence: 'a skier in the midst of a jump'. Replace the letter 'm' with 'd' in the sentence.
Example Output: a skier in the didst of a judp

Example Input: Sentence: 'a car travelling on the street near a traffic light and traffic signs'. Replace the letter 'g' with 'f' in the sentence.
Example Output:
a car travellinf on the street near a traffic lifht and traffic sifns