Remove mapping from ft=cpp to c++

[phcerdan]

In my zeal v.0.4.0 the C++ docs are associated to the string cpp not c++ so this mapping is not necessary.

[phcerdan]

I can solve it meanwhile with:

let g:zv_file_types = {
      \ 'cpp'                : 'cpp',
      \  }

[KabbAmine]

Make sense but please update the README and the docfile accordingly, and squash + force push it.

[KabbAmine]

Thanks for your contribution.