exercises 02_basic_calculator/04_panics: fix comma splice in panic message

exercises/02_basic_calculator/04_panics/src/lib.rs

@@ -19,7 +19,7 @@ mod tests {
     // 👇 With the `#[should_panic]` annotation we can assert that we expect the code
     //    under test to panic. We can also check the panic message by using `expected`.
     //    This is all part of Rust's built-in test framework!
-    #[should_panic(expected = "The journey took no time at all, that's impossible!")]
+    #[should_panic(expected = "The journey took no time at all. That's impossible!")]
     fn by_zero() {
         speed(0, 10, 0);
     }