update Dockerfile

README.md

@@ -123,9 +123,9 @@ FROM openjdk
 COPY src hello-world/src
 WORKDIR hello-world
 RUN mkdir -p bin
-RUN javac -d bin ./main.java
+RUN javac -d bin ./src/main.java
 WORKDIR bin
-CMD ["java", "main.java"]
+CMD ["java", "main"]
 ```
 
 **Note**: Make sure that the scripts is named exactly as `Dockerfile` or `dockerfile`.