Avoid copying extraneous files in opam-devel example

[maroneze]

I just did opam install opam-devel to upgrade from 2.0.4 to 2.0.5, and in the end there was this message:

You should not run it from there, please install the binaries to your PATH, e.g. with
sudo cp <OPAMROOT>/4.05.0/lib/opam-devel/* /usr/local/bin

I know it's just an example, but that command would actually copy the META and dune-package files as well. And since there is only a single binary there (opam), it seems better to just do:

   sudo cp <OPAMROOT>/4.05.0/lib/opam-devel/opam /usr/local/bin

Unless there is a use case in which opam-devel has more than one binary file. If so, please ignore this PR.

[AltGr]

Ah, indeed: there used to be a opam-admin there too (and no META or other dune artifacts). But this no longer makes much sense. Thanks!

There might be a need for a .exe here in some cases but well... :)

[dra27]

There might be a need for a .exe here in some cases but well... :)

The fact there's no /usr/local/bin would be a much bigger concern!