fixed DECam config file

[igorandreoni]

I made these fixes:

• the available filters are ugrizy, not only r
• horizon key was repeated
• slew rate and overheads were incorrect

[mcoughlin]

@igorandreoni Looks like the test output files need to be updated with this. https://github.com/skyportal/gwemopt/blob/main/gwemopt/tests/test_schedule.py#L62 @robertdstein had left an example here.