[pairs.pair] Missed rename from U,V to U1,U2. (#2037)

Author: jensmaurer

source/utilities.tex

@@ -719,8 +719,8 @@
 \begin{itemdescr}
 \pnum
 \effects
-Assigns to \tcode{first} with \tcode{std::forward<U>(p.first)}
-and to \tcode{second} with\\ \tcode{std::forward<V>(p.second)}.
+Assigns to \tcode{first} with \tcode{std::forward<U1>(p.first)}
+and to \tcode{second} with\\ \tcode{std::forward<U2>(p.second)}.
 
 \pnum
 \remarks This operator shall not participate in overload resolution unless