Fix Doc for Dependent Scheduler

The doc says: 

```
Note that, for SourceStamps generated by a ChangeSource, the ``revision`` is ``None``, meaning HEAD.
If any changes are committed between the time the upstream scheduler begins its build and the time the dependent scheduler begins its build, then those changes will be included in the downstream build.
See the :ref:`Triggerable-Scheduler` for a more flexible dependency mechanism that can avoid this problem.
```
To me, it should be: 

```
Note that, for SourceStamps generated by a Dependent Scheduler, the ``revision`` is ``None``, meaning HEAD.
If any changes are committed between the time the upstream scheduler begins its build and the time the dependent scheduler begins its build, then those changes will be included in the downstream build.
```