Fix NullableAttribute.Flags->NullableFlags in docs (#36907)

docs/features/nullable-metadata.md

@@ -23,11 +23,11 @@ namespace System.Runtime.CompilerServices
         public readonly byte[] NullableFlags;
         public NullableAttribute(byte flag)
         {
-            Flags = new byte[] { flag };
+            NullableFlags = new byte[] { flag };
         }
         public NullableAttribute(byte[] flags)
         {
-            Flags = flags;
+            NullableFlags = flags;
         }
     }
 }