fix(rule): use const instead of var

Author: azu

src/no-todo.js

@@ -36,8 +36,8 @@ export default function (context) {
             - [ ] Todo
         */
         [Syntax.ListItem](node) {
-            var text = context.getSource(node);
-            var match = text.match(/\[\s+\]\s/i);
+            const text = context.getSource(node);
+            const match = text.match(/\[\s+\]\s/i);
             if (match) {
                 report(node, new context.RuleError(`Found TODO: '${text}'`, {
                     index: match.index