docs: update link to help-wanted issues (#299)

Co-authored-by: Christian Bewernitz <coder@karfau.de>

readme.md

@@ -6,7 +6,7 @@
 [![license](https://img.shields.io/npm/l/@xmldom/xmldom?color=blue&style=flat-square)](LICENSE)
 [![npm](https://img.shields.io/npm/v/@xmldom/xmldom?style=flat-square)](https://www.npmjs.com/package/@xmldom/xmldom)
 [![bug issues](https://img.shields.io/github/issues/xmldom/xmldom/bug?color=red&style=flat-square)](https://github.com/xmldom/xmldom/issues?q=is%3Aissue+is%3Aopen+label%3Abug)
-[!["help wanted" issues](https://img.shields.io/github/issues/xmldom/xmldom/help%20wanted?color=darkgreen&style=flat-square)](https://github.com/xmldom/xmldom/issues?q=is%3Aissue+is%3Aopen+label%3A%22help+wanted%22)
+[![help-wanted issues](https://img.shields.io/github/issues/xmldom/xmldom/help-wanted?color=darkgreen&style=flat-square)](https://github.com/xmldom/xmldom/issues?q=is%3Aissue+is%3Aopen+label%3Ahelp-wanted)
 [![Mutation report](https://img.shields.io/endpoint?style=flat-square&url=https%3A%2F%2Fbadge-api.stryker-mutator.io%2Fgithub.com%2Fxmldom%2Fxmldom%2Fmaster)](https://dashboard.stryker-mutator.io/reports/github.com/xmldom/xmldom/master)