[docs] Fix Kaleidoscope code example

* replace virtual with override
* use default like in full code example

Differential Revision: https://reviews.llvm.org/D123110

llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl03.rst

@@ -34,7 +34,7 @@ class:
     /// ExprAST - Base class for all expression nodes.
     class ExprAST {
     public:
-      virtual ~ExprAST() {}
+      virtual ~ExprAST() = default;
       virtual Value *codegen() = 0;
     };
 
@@ -44,7 +44,7 @@ class:
 
     public:
       NumberExprAST(double Val) : Val(Val) {}
-      virtual Value *codegen();
+      Value *codegen() override;
     };
     ...