[default.allocator.general] Fix indentation (#7075)

Author: Andreas-Krug

source/memory.tex

@@ -1673,7 +1673,7 @@
 \begin{codeblock}
 namespace std {
   template<class T> class allocator {
-   public:
+  public:
     using value_type                             = T;
     using size_type                              = size_t;
     using difference_type                        = ptrdiff_t;