Use the actual PIP package name

[pschmitt]

The upstream package name is pynvim, not neovim. It's an alternative package name and therefore isn't the one returned by pip freeze. This PR does not change any of the functionality of this plugin.

See:

• https://github.com/neovim/pynvim/blob/master/setup.py#L35
• https://github.com/neovim/pynvim/commit/f2372385d1be6ed62ee458d5e017fff3541ce0f7

source ~/.local/share/nvim/vim-plug/python-support.nvim/autoload/nvim_py3/bin/activate
pip freeze | grep vim
# pynvim==0.4.0