fixing sphinx errors

fixing sphinx warnings/errors.

```
% make docs                                                                                       -I-
make -C master/docs
make[1]: Entering directory `/home/bak1an/workspaces/buildbot/buildbot/master/docs'
rm -rf _build/*
sphinx-build -b html -d _build/doctrees  -q -W . _build/html
Making output directory...

Warning, treated as error:
/home/bak1an/workspaces/buildbot/buildbot/master/docs/manual/cfg-buildsteps.rst:4: SEVERE: Duplicate ID: "step-Darcs".

make[1]: *** [html] Error 1
make[1]: Leaving directory `/home/bak1an/workspaces/buildbot/buildbot/master/docs'
make: *** [docs] Error 2
```