Update Memory<T>: Link for "ref struct" is wrong (dotnet#8581)

The link to "ref struct" linked to the normal struct page, not the ref struct page.

Author: colejohnson66

xml/System/Memory`1.xml

@@ -60,7 +60,7 @@
   
 ## Remarks  
   
-Like <xref:System.Span%601>, `Memory<T>` represents a contiguous region of memory. Unlike <xref:System.Span%601>, however, `Memory<T>` is not a [ref struct](/dotnet/csharp/language-reference/builtin-types/struct#ref-struct). This means that `Memory<T>` can be placed on the managed heap, whereas <xref:System.Span%601> cannot. As a result, the `Memory<T>` structure does not have the same restrictions as a <xref:System.Span%601> instance. In particular:
+Like <xref:System.Span%601>, `Memory<T>` represents a contiguous region of memory. Unlike <xref:System.Span%601>, however, `Memory<T>` is not a [ref struct](/dotnet/csharp/language-reference/builtin-types/ref-struct). This means that `Memory<T>` can be placed on the managed heap, whereas <xref:System.Span%601> cannot. As a result, the `Memory<T>` structure does not have the same restrictions as a <xref:System.Span%601> instance. In particular:
 
 - It can be used as a field in a class.