fix Dockerfile.example

[adrelino]

This fixes #5 in upstream: Docker example image for frontend won't build

[CLAassistant]

All committers have signed the CLA.

[adrelino]

it is only 1 character change, please review and merge so I can start my next pull request

[ingoaf]

@adrelino Today, I brought the topic of not reviewing the PRs up in our internal Siemens forum. I will come back to you as soon as there are any updates. Thanks for your contribution so far! :)

[ingoaf]

@adrelino following up as promised: I talked to @Milan-Hurban and he told me that

• there is currently a discussion about software clearing with @HummelN7872 - @Milan-Hurban will check the status and clarify in which way the development of the connectivity tester will continue
• there is a new branch (https://github.com/industrial-edge/connectivity-tester/tree/dev-milan) which will make my PR (functionality of S7 scanner) and your PR (python3 package fix) obsolete

@Milan-Hurban if I understood you correctly could you please close both open PRs?

In any case, like I said before: thank you very much for your contribution and I hope we can move forward with the development soon.