docs: markdownItFactory is not necessary

Ref: https://github.com/theoludwig/markdownlint-rule-relative-links/issues/13

.markdownlint-cli2.mjs

@@ -1,4 +1,4 @@
-import relativeLinksRule, { markdownIt } from "./src/index.js"
+import relativeLinksRule from "./src/index.js"
 
 const config = {
   config: {
@@ -10,9 +10,6 @@ const config = {
   globs: ["**/*.md"],
   ignores: ["**/node_modules", "**/test/fixtures/**"],
   customRules: [relativeLinksRule],
-  markdownItFactory: () => {
-    return markdownIt
-  },
 }
 
 export default config