[util.smartptr.shared] Fix punctuation in synopsis comment.

Author: Dawn Perchik

source/utilities.tex

@@ -8818,7 +8818,7 @@
     template<class U> bool owner_before(weak_ptr<U> const& b) const;
   };
 
-  // \ref{util.smartptr.shared.create}, shared_ptr creation
+  // \ref{util.smartptr.shared.create}, shared_ptr creation:
   template<class T, class... Args> shared_ptr<T> make_shared(Args&&... args);
   template<class T, class A, class... Args>
     shared_ptr<T> allocate_shared(const A& a, Args&&... args);