Fix backend usage in DBI intro notebook

[MatteoRobbiati]

Checklist:

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.84%. Comparing base (45727e0) to head (10f559c).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1378   +/-   ##
=======================================
  Coverage   99.84%   99.84%           
=======================================
  Files          76       76           
  Lines       10844    10844           
=======================================
  Hits        10827    10827           
  Misses         17       17           

Flag	Coverage Δ
unittests	99.84% <ø> (ø)

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[andrea-pasquale]

Thanks @MatteoRobbiati, could you push the notebook not executed?