docs: clarify build must run from core directory

Author: rexminnis

CONTRIBUTING.md

@@ -36,4 +36,5 @@ RayDP is a mixed-language project (Scala + Python). To set up your development e
 To verify your setup, run the build from the terminal (or the Maven side panel):
 
 ```bash
+cd core
 mvn clean package -DskipTests