Fix capitalisation

Author: dannyadair

README.md

@@ -84,7 +84,7 @@ pdfbaker bake examples/examples.yaml
 
 ## Development
 
-All source code is [on github](https://github.com/pythonnz/pdfbaker).
+All source code is [on GitHub](https://github.com/pythonnz/pdfbaker).
 
 This project uses [uv](https://github.com/astral-sh/uv) for dependency management. The
 `uv.lock` file ensures reproducible builds.