.NGRP not referenced as an alias in special-symbols.RD

Just noticed that `.NGRP` is not an alias in [special-symbols.Rd](https://github.com/Rdatatable/data.table/blob/63841477b5f23215e33dc9d420ee45c43ea41e97/man/special-symbols.Rd).

Should it be?

```R
> ?`.EACHI`
> ?`.GRP`
> ?`.NGRP`
No documentation for ‘.NGRP’ in specified packages and libraries:
you could try ‘??.NGRP’
```