Incorrect reference to AbstractJobTests in documentation

[fmbenhassine]

The Testing Individual Steps section of the reference documentation refers to a non existent class AbstractJobTests:

[...]
The AbstractJobTests class contains a method called launchStep, which takes a step name and runs just that particular Step.
[...]

AbstractJobTests should be replaced with JobLauncherTestUtils.

[lixiangbetter]

If no one is in charge, i'd like to do it! This will be my first contribution for this project.