Fix typo in README.md

Author: nblumhardt

README.md

@@ -387,7 +387,7 @@ User-defined functions are supplied through an instance of `NameResolver`:
 
 ```csharp
 var myFunctions = new StaticMemberNameResolver(typeof(MyFunctions));
-var expr = SerilogExpression.Compile("IsHello(User.Name)", nameResolver: customSerilogFunctions);
+var expr = SerilogExpression.Compile("IsHello(User.Name)", nameResolver: myFunctions);
 // Filter events based on whether `User.Name` is `'Hello'` :-)
 ```