Corrected the license URL for the ASF ASL 2.0 license.

Running the Maven License Plugin fails for assertJ with an error message since the license URL points to a local file:

    mvn license:download-licenses
    ...
    [INFO] --- license-maven-plugin:1.7:download-licenses (default-cli) @ clienttesting ---
    [WARNING] POM for dependency org.assertj:assertj-core has an invalid license URL: LICENSE.txt  
  
With this commit the license URL points to a valid URL for the ASF AL 2.0.