Fixed typo in VimtexCountWords' help
Fixed typo in VimtexCountWords' help

The command `VimtexCountWords`, `VimtexCountLetters` (and the ! variants) run `texcount -nosub -sum`, not `texcount -nosub -sub`. 
The latter would, incidentally, negate the usage of `-nosub`
lervag/vimtex,kmarius/vimtex,lervag/vimtex,kmarius/vimtex,Aster89/vimtex,Aster89/vimtex