docs: update README.md

Author: MichaelCurrin

README.md

@@ -4,7 +4,7 @@
 [![GitHub tag](https://img.shields.io/github/tag/MichaelCurrin/code-cookbook?include_prereleases&sort=semver)](https://github.com/MichaelCurrin/code-cookbook/releases/)
 [![License](https://img.shields.io/badge/License-MIT-blue)](#license)
 
-[![Jekyll - 3.9](https://img.shields.io/badge/Jekyll-3.9-blue?logo=jekyll&logoColor=white)](https://jekyllrb.com)
+[![Jekyll](https://img.shields.io/badge/Jekyll-4-blue?logo=jekyll&logoColor=white)](https://jekyllrb.com)
 [![theme - fractal](https://img.shields.io/static/v1?label=theme&message=fractal&color=blue&logo=github)](https://github.com/MichaelCurrin/fractal)
 [![Hosted with - GitHub Pages](https://img.shields.io/badge/Hosted_with-GitHub_Pages-blue?logo=github&logoColor=white)](https://pages.github.com/)