eval.c: Rename has('neovim') feature to has('nvim')

To follow the convention of 'nvim' for technical references

runtime/doc/nvim_python.txt

@@ -30,7 +30,7 @@ simple step-by-step:
 - Add the following snippet to your `vimrc`, before any python plugins are
   loaded:
 >
-  if has('neovim')
+  if has('nvim')
     runtime! plugin/python_setup.vim
   endif
 <