Change "typename decay<F>::type" to "decay_t<F>"

Looks like this one was overlooked when `decay_t` (and the other `_t` helpers) were introduced.

Author: d-frey

source/utilities.tex

@@ -8859,7 +8859,7 @@
 \pnum\returns \tcode{*this}
 
 \pnum\remarks This assignment operator shall not participate in overload
-resolution unless \tcode{declval<typename decay<F>::type\&>()} is
+resolution unless \tcode{declval<decay_t<F>\&>()} is
 Callable~(\ref{func.wrap.func}) for argument types \tcode{ArgTypes...} and
 return type \tcode{R}.
 \end{itemdescr}