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

Example Input: Sentence: 'a person on a surfboard riding in ocean waves'. Replace the letter 's' with 'g' in the sentence.
Example Output: a pergon on a gurfboard riding in ocean waveg

Example Input: Sentence: 'a gray elephant standing next to green bushes'. Replace the letter 'e' with 'o' in the sentence.
Example Output: a gray olophant standing noxt to groon bushos

Example Input: Sentence: 'two gray and white  cats sleeping next to each other in a pink blanket'. Replace the letter 'l' with 'u' in the sentence.
Example Output:
two gray and white  cats sueeping next to each other in a pink buanket