fix broken link to issues in CONTRIBUTING.md

Changed relative link "issues" to absolute URL "https://github.com/docsifyjs/docsify/issues" to prevent 404.

Author: Gilbertode-iden254

CONTRIBUTING.md

@@ -9,7 +9,7 @@ We welcome any type of contribution, not only code. You can help with
 - **QA**: file bug reports, the more details you can give the better (e.g. screenshots with the console open)
 - **Marketing**: writing blog posts, howto's, printing stickers, ...
 - **Community**: presenting the project at meetups, organizing a dedicated meetup for the local community, ...
-- **Code**: take a look at the [open issues](issues). Even if you can't write code, commenting on them, showing that you care about a given issue matters. It helps us triage them.
+- **Code**: take a look at the [open issues](https://github.com/docsifyjs/docsify/issues).. Even if you can't write code, commenting on them, showing that you care about a given issue matters. It helps us triage them.
 - **Money**: we welcome financial contributions in full transparency on our [open collective](https://opencollective.com/docsify).
 
 ## Your First Contribution