Typo (?) Rf_lengthgets should be Rf_xlengthgets

[MichaelChirico]

https://github.com/r-lib/brio/blob/07de917a99d1f10626a431ad124ca71e33b43d60/src/read_lines.c#L136

I see the comment about mentions xlengthgets:

https://github.com/r-lib/brio/blob/07de917a99d1f10626a431ad124ca71e33b43d60/src/read_lines.c#L133

And also the previous implementation used xlength:

https://github.com/r-lib/brio/blob/c157ff766c307756b43032a62c5aa0136f5da65c/src/read_lines.c#L133

So I wonder if the comment is wrong, or if xlengthgets was intended.