Update standard/expressions.md

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

Author: Nigel-Ecma

standard/expressions.md

@@ -1774,7 +1774,7 @@ A  *null_conditional_member_access* expression `E` is of the form `P?.A`. The me
   - Otherwise the type of `E` is `T`, and the meaning of `E` is the same as the meaning of:
 
     ```csharp
-    ((object)P == null) ? (T)null : P.GetValueOrDefault().A
+    ((object)P == null) ? (T)null : P.Value.A
     ```
 
     Except that `P` is evaluated only once.