Outdated reference to utf16 Data.Text in docs

[sullyj3]

unbeliever/core-text/lib/Core/Text/Rope.hs

Lines 42 to 46 in c358dd2

	same design; it is likewise backed by arrays. Rather surprisingly, the storage
	backing Text objects are encoded in UTF-16, meaning every time you want to
	work with unicode characters that came in from /anywhere/ else and which
	inevitably are UTF-8 encoded you have to convert to UTF-16 and copy into a new
	array, wasting time and memory.

I believe this is out of date as of haskell/text#365

[istathar]

Yeah, I've been waiting for text 2.0 to propagate. Once people are more widely able to use GHC 9.0 and lts-19.x and we're able to remove the conversion step that'll be great. You're right, though, this comment is now dated.