Build instructions result in build errors

[dthaler]

https://github.com/iovisor/ubpf#checking-out says:

git submodule init
git submodule update --recursive

However that only does an init at the top level, not recursively, and as a result elfio is not initialized and so the build fails.

Instead the two lines should be replaced with one line:

git submodule update --init --recursive

Which indeed is what the CICD actually does.