You can fix spelling error by selecting the first suggested alternative automagically using
1z=. Add this to your (Neo)Vim config, and you will thank me later:
inoremap <C-s> <c-g>u<Esc>[s1z=`]a<c-g>u
(Or bind whatever key you want.) This allows to fix any previous error(s) (thanks to the last bit of magic) using the first suggested entry while you’re in insert mode.