Fix API reference link to point to monorepo (works in split repo too)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Prophet731

README.md

@@ -155,7 +155,7 @@ try {
 
 ## API Reference
 
-See [API_REFERENCE.md](../API_REFERENCE.md) for the complete VirtFusion API specification.
+See the [API Reference](https://github.com/EZSCALE/virtfusion-api-library/blob/main/API_REFERENCE.md) for the complete VirtFusion API specification.
 
 ## License