[DOC] how-to-build: ubuntu use pip install pillow [MANUAL MERGE]

@lilafisch use ubuntu and found this "bug" in the docu

Looks good to me! Thanks for keeping our codebase clean.

However, I'm only a bot - so a human will still have to approve this request.

mentioned in commit 872e900b

changed title from [DOC] how-to-build: ubuntu use pip install pillow to [DOC] how-to-build: ubuntu use pip install pillow [MANUAL MERGE]

Manually merged in 872e900b. Thanks!

closed