Change the return type of monotonic_buffer_resource's deleted assignment

... to monotonic_buffer_resource& for consistency with the default definition of the builtin assignment operator. This change has no normative effect, it only eliminates the inconsistency as a potential source of confusion.

Since the return type of deleted functions is not observable, this change is editorial.

Author: CaseyCarter

source/utilities.tex

@@ -11944,7 +11944,7 @@
 
   virtual ~monotonic_buffer_resource();
 
-  monotonic_buffer_resource
+  monotonic_buffer_resource&
     operator=(const monotonic_buffer_resource&) = delete;
 
   void release();