Function replace_char 

pub fn replace_char(string: &str, index: i32, replacement: char) -> String

Replaces the character at the given index and returns the string.

Example:

 let input: &str = "Hello, World!";
 let result: String = funny_string::replace_char(input, 3, 'X');
 // result now contains the string "HelXo, World!"