[Rd] Small typo in Writing R Extensions

[Davis Vaughan]

Hi all,

Writing R Extensions describes `R_NewEnv()` as:

```
At times it may also be useful to create a new environment frame in C
code. R_NewEnv is a C version of the R function new.env:

SEXP R_NewEnv(SEXP enclos, int hash, ins size)
```

There is a typo here where `ins size` should be `int size`.

Thanks!
Davis