Fix tutorial example for how to test with a script.

The script must exit with 0 if there is not problem and non-zero otherwise.
Since `grep` exits with 0 if a line matches the pattern we need to negate its
exit code.

Author: orium

TUTORIAL.md

@@ -222,7 +222,7 @@ previous interactive session can be hands-free automated with this script:
 `foo/test.sh`:
 ```sh
 #!/bin/sh
-cargo check 2>&1 | grep E0642
+! cargo check 2>&1 | grep E0642
 ```
 
 And then run: