Update index.md

Fix index.js path to avoid error thrown at startup

docs/tutorial/our-application/index.md

@@ -43,7 +43,7 @@ see a few flaws in the Dockerfile below. But, don't worry! We'll go over them.
     WORKDIR /app
     COPY . .
     RUN yarn install --production
-    CMD ["node", "src/index.js"]
+    CMD ["node", "./src/index.js"]
     ```
 
     Please check that the file `Dockerfile` has no file extension like `.txt`. Some editors may append this file extension automatically and this would result in an error in the next step.