[unique.ptr.runtime.asgn] Fix missing space (#4023)

Author: FrankHB

source/utilities.tex

@@ -9223,7 +9223,7 @@
 
 \indexlibrarymember{operator=}{unique_ptr}%
 \begin{itemdecl}
-template<class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u)noexcept;
+template<class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u) noexcept;
 \end{itemdecl}
 
 \begin{itemdescr}