[allocator.requirements.general] Fix SimpleAllocator example (#6152)

The example now meets the requirements and is minimal.
Previously, some == comparisons that should work were ambiguous.

Author: blackteahamburger

source/lib-intro.tex

@@ -2737,10 +2737,10 @@
 
   template<class U> SimpleAllocator(const SimpleAllocator<U>& other);
 
-  [[nodiscard]] T* allocate(std::size_t n);
+  T* allocate(std::size_t n);
   void deallocate(T* p, std::size_t n);
 
-  bool operator==(const SimpleAllocator&) const;
+  template<class U> bool operator==(const SimpleAllocator<U>& rhs) const;
 };
 \end{codeblock}
 \end{example}