Fix instructions for clingwrapper installation

[c-w]

As discussed on 172, installing clingwrapper from source fails with the following stacktrace:

src/clingwrapper.cxx:2001:26: error: no member named 'GetTagDeclId' in 'CppyyLegacy::TDataMember'; did you mean 'GetDeclId'?
          auto declid = m->GetTagDeclId();
                           ^~~~~~~~~~~~
                           GetDeclId

Updating the installation instructions to not use PEP 517 fixes the setup.