Error in guide to dockerizing nodejs

* **URL**: https://nodejs.org/en/docs/guides/nodejs-docker-webapp/

The following instruction, given by the above tutorial, causes a build to fail.
`COPY package.json package-lock.json .`

The error message reads:
```
Step 3/7 : COPY ./package.json ./package-lock.json .
When using COPY with more than one source file, the destination must be a directory and end with a /
```