Fix imports in 'slightly more complex circuit'

[rscircus]

In this example qibo is accessed directly in

# execute on quantum hardware
qibo.set_backend("qibolab", "dummy")

therefore an import of qibo is needed.

Checklist:

• [x] Reviewers confirm new code works as expected.
• [x] Tests are passing.
• [x] Coverage does not decrease.
• [x] Documentation is updated.

[rscircus]

Thx for the quick review. Am sick today and thought I set everything up to work through the docs (in slo-mo). :-) Nice project.