Update standard/expressions.md

Co-authored-by: Jon Skeet <[email protected]>

Author: Nigel-Ecma

standard/expressions.md

@@ -2137,7 +2137,7 @@ A *null_conditional_element_access* expression `E` is of the form `P?[A]B`; wher
   - Otherwise the type of `E` is `T`, and the meaning of `E` is the same as the meaning of:
 
     ```csharp
-    ((object)P == null) ? null : P.GetValueOrDefault()[A]B
+    ((object)P == null) ? null : P.Value[A]B
     ```
 
     Except that `P` is evaluated only once.