chore: fix implementation type definition

Signed-off-by: Vincent Biret <[email protected]>

Author: baywet

src/Microsoft.OpenApi/Models/References/BaseOpenApiReferenceHolder.cs

@@ -72,7 +72,7 @@ protected BaseOpenApiReferenceHolder(string referenceId, OpenApiDocument? hostDo
 
 #if NETSTANDARD2_1_OR_GREATER
     /// <inheritdoc/>
-    public required U Reference { get; init; }
+    public required V Reference { get; init; }
 #else
     /// <inheritdoc/>
     public V Reference { get; init; }