Use decay_t<> rather than typename decay<>::type. (#979)

Author: W-E-Brown

source/iostreams.tex

@@ -10828,7 +10828,7 @@
   function argument \tcode{const Source\&} \tcode{source} shall
   have an effective range \range{source}{end} where
   \tcode{end} is the first iterator value with an element value equal to
-  \tcode{iterator_traits<decay<Source>:\-:type>::value_type()}.
+  \tcode{iterator_traits<decay_t<Source>>::value_type()}.
 \end{itemize}
 
 \pnum