Minor spelling correction. Altered description of how `data.table` optim...

...izes calls to `mean`

`.External(Cfastmean, B, FALSE)` is used instead of `.Internal(mean(x))`. See https://r-forge.r-project.org/scm/viewvc.php/pkg/src/fastmean.c?view=markup&root=datatable for details. fastmean is (almost) identical to the base R implementation, except that it deals with `NA` values within the C code, not within R.