[allocator.requirements.general] Add namespace std for exposition-only concept

Author: hewillk

source/lib-intro.tex

@@ -2958,14 +2958,16 @@
 The following exposition-only concept defines
 the minimal requirements on an Allocator type.
 \begin{codeblock}
-template<class Alloc>
-concept @\defexposconcept{simple-allocator}@ =
-  requires(Alloc alloc, size_t n) {
-    { *alloc.allocate(n) } -> @\libconcept{same_as}@<typename Alloc::value_type&>;
-    { alloc.deallocate(alloc.allocate(n), n) };
-  } &&
-  @\libconcept{copy_constructible}@<Alloc> &&
-  @\libconcept{equality_comparable}@<Alloc>;
+namespace std {
+  template<class Alloc>
+  concept @\defexposconcept{simple-allocator}@ =
+    requires(Alloc alloc, size_t n) {
+      { *alloc.allocate(n) } -> @\libconcept{same_as}@<typename Alloc::value_type&>;
+      { alloc.deallocate(alloc.allocate(n), n) };
+    } &&
+    @\libconcept{copy_constructible}@<Alloc> &&
+    @\libconcept{equality_comparable}@<Alloc>;
+}
 \end{codeblock}
 A type \tcode{Alloc} models \exposconcept{simple-allocator}
 if it meets the requirements of \ref{allocator.requirements.general}.