[mem.res] Fix namespace for definitions in std::pmr

The previous commit ad767d3 accidentally added the wrong namespace "std" instead of "std::pmr".

Author: tkoeppe

source/utilities.tex

@@ -10856,7 +10856,7 @@
 
 \indexlibrary{\idxcode{memory_resource}}%
 \begin{codeblock}
-namespace std {
+namespace std::pmr {
   class memory_resource {
     static constexpr size_t max_align = alignof(max_align_t);   // \expos
 
@@ -11023,7 +11023,7 @@
 
 \indexlibrary{\idxcode{polymorphic_allocator}}%
 \begin{codeblock}
-namespace std {
+namespace std::pmr {
   template <class Tp>
     class polymorphic_allocator {
       memory_resource* memory_rsrc;     // \expos
@@ -11535,7 +11535,7 @@
 \indexlibrary{\idxcode{synchronized_pool_resource}}%
 \indexlibrary{\idxcode{unsynchronized_pool_resource}}%
 \begin{codeblock}
-namespace std {
+namespace std::pmr {
   struct pool_options {
     size_t max_blocks_per_chunk = 0;
     size_t largest_required_pool_block = 0;
@@ -11838,7 +11838,7 @@
 
 \indexlibrary{\idxcode{monotonic_buffer_resource}}%
 \begin{codeblock}
-namespace std {
+namespace std::pmr {
   class monotonic_buffer_resource : public memory_resource {
     memory_resource *upstream_rsrc;     // \expos
     void *current_buffer;               // \expos