[numerics.sat.cast] Rename template parameters to "R" and "T".

This makes the names consistent with std::in_range in <utility>.

Author: tkoeppe

source/algorithms.tex

@@ -10867,19 +10867,19 @@
 
 \indexlibraryglobal{saturate_cast}%
 \begin{itemdecl}
-template<class T, class U>
-  constexpr T saturate_cast(U x) noexcept;
+template<class R, class T>
+  constexpr R saturate_cast(T x) noexcept;
 \end{itemdecl}
 
 \begin{itemdescr}
 \pnum
 \constraints
-\tcode{T} and \tcode{U} are signed or unsigned integer types\iref{basic.fundamental}.
+\tcode{R} and \tcode{T} are signed or unsigned integer types\iref{basic.fundamental}.
 
 \pnum
 \returns
-If \tcode{x} is representable as a value of type \tcode{T}, \tcode{x};
-otherwise, either the largest or smallest representable value of type \tcode{T},
+If \tcode{x} is representable as a value of type \tcode{R}, \tcode{x};
+otherwise, either the largest or smallest representable value of type \tcode{R},
 whichever is closer to the value of \tcode{x}.
 \end{itemdescr}