Wrong src-dir and test-dir for Maven

These patterns:
https://github.com/bbatsov/projectile/blob/df3d73e1f0ff625a09196ff3ba6f4be82a53fb3b/projectile.el#L3119-L3120

should instead be `src/main` and `src/test`, according to Maven [documentation](https://maven.apache.org/guides/introduction/introduction-to-the-standard-directory-layout.html).

Sorry for not going through the template, I'm on the clock. :smile:
