Fix documentation typo: gratesettings => grate_settings

[salticus]

Minor fix to documentation typo.

In docs/ConfigurationOptions/ResponseFiles.md the example response file is named first grate_settings.rsp then gratesettings.rsp. Testing on Ubuntu showed that this would result in the following error:

Response file not found './gratesettings.rsp'.

(Not being familiar with ResponseFiles, this seemed worth testing, in case there was some kind of "remove all underscores in a filename" magic going on.)

[erikbra]

Thanks a lot for you contribution! Documentation fixes are some of the most important ones. If the docs are incorrect, nothing works!

That said, please don't build too much infrastructure on top of the .rsp feature. I'm considering migrating to another command-line parsing library, as System.CommandLine never seems to go out of beta (and I think it never will, in the current form). So the .rsp support might disappear in the future 😄