chore: Change point to comma in from_into.rs

A typo in the fn test_bad_age() hint message had a point rather than comma

Prev:
// Test that "Mark.twenty"

Current:
// Test that "Mark,twenty"

exercises/conversions/from_into.rs

@@ -75,7 +75,7 @@ mod tests {
     }
     #[test]
     fn test_bad_age() {
-        // Test that "Mark.twenty" will return the default person due to an error in parsing age
+        // Test that "Mark,twenty" will return the default person due to an error in parsing age
         let p = Person::from("Mark,twenty");
         assert_eq!(p.name, "John");
         assert_eq!(p.age, 30);