Clean up README

[bparks13]

See the following line as an example ["that that", "completely successfully" => "complete successfully"].

https://github.com/open-ephys/onix1-bonsai-docs/blob/99a50ed0e87cd97da671ef89bc6605ec0e7ec29b/README.md?plain=1#L77

Check that no other errors exist.

[cjsha]

jonnew mentioned that we should add a note about how the source code is accessible in the docs repo as a submodule. I think this was a point of confusion for him at first. I forgot to add this when he said it to me.

[cjsha]

We should also add some notes about a script for cleaning old files once that issue is resolved.