[range.prim.empty] Fix misapplication of P2091R0

Replace "Given a subexpression ranges::empty(E)" with the intended wording "Given a subexpression E".

Author: onihusube

source/ranges.tex

@@ -792,7 +792,7 @@
 object\iref{customization.point.object}.
 
 \pnum
-Given a subexpression \tcode{ranges::empty(E)} with type \tcode{T},
+Given a subexpression \tcode{E} with type \tcode{T},
 let \tcode{t} be an lvalue that denotes the reified object for \tcode{E}.
 Then: