[rand.dist.samp.plinear] Fix copy & paste error in Mandates (#6794)

This error was present in the incoming P1719R2 paper (Mandating the
Standard Library: Clause 26 - Numerics Library), but is obviously bogus.
There is no UnaryOperation type in that constructor. The correct
requirement is taken from the corresponding constructor in
[rand.dist.samp.pconst].

Author: jwakely

source/numerics.tex

@@ -6395,7 +6395,12 @@
 \begin{itemdescr}
 \pnum
 \mandates
-\tcode{is_invocable_r_v<double, UnaryOperation\&, double>} is \tcode{true}.
+Both of
+\begin{itemize}
+\item{\tcode{is_convertible_v<iterator_traits<InputIteratorB>::value_type, double>}}
+\item{\tcode{is_convertible_v<iterator_traits<InputIteratorW>::value_type, double>}}
+\end{itemize}
+are \tcode{true}.
 
 \pnum
 \expects