fix numbering sequence

exercise/testing/src/lib.rs

@@ -32,7 +32,7 @@ mod test {
     // - splish(9, 3) is 0
 }
 
-// 4. Create a `tests/` directory and an integration test file `tests/more_tests.rs`
+// 5. Create a `tests/` directory and an integration test file `tests/more_tests.rs`
 // Inside that file, create a test function that verifies:
 // - that `sploosh(splish(-1, 0), splish(1, 1), splish(3, 2))` returns the value `4`
 //