docs: update visitor demo

Author: HaydenOrz

README-zh_CN.md

@@ -180,8 +180,9 @@ class MyVisitor extends AbstractParseTreeVisitor<Result> implements MySqlParserV
         console.log('SelectElements', selectElements);
         return '';
     }
-    visitProgram(ctx) {
-        return 'Return by program node'
+    visitProgram(ctx) { // program 是根规则
+        this.visitChildren(ctx);
+        return 'Return by program context'
     }
 }
 const visitor = new MyVisitor();

README.md

@@ -187,8 +187,9 @@ class MyVisitor extends AbstractParseTreeVisitor<Result> implements MySqlParserV
         console.log('SelectElements:', selectElements);
         return '';
     }
-    visitProgram(ctx) {
-        return 'Return by program node'
+    visitProgram(ctx) { // program is root rule
+        this.visitChildren(ctx);
+        return 'Return by program context'
     }
 }
 const visitor = new MyVisitor();