Bug: documentation for --dest and --dest-dir does not match actual behavior

[protz]

When one of the arguments passed to cargo is --manifest-path, the --dest-dir and --dest options no longer behave according to the documentation.

Specifically, charon --dest-file foo.llbc -- --manifest-path path/to/Cargo.toml creates path/to/foo.llbc even though the option documentation states that ./foo.llbc will be created.

My preferred fix would be for the options to behave as stated in the documentation, /even when/ --manifest-path is used, so as to avoid running an additional mv in the Makefile (not to mention that if the proofs are in directory that's separate from the main code repository, it's not super clean to write build outputs outside of the proof project)