Fix dispersive shift plot

[andrea-pasquale]

Closes #646.
There was a factor 1e9 wrong (possibly forgotten in #627)..

Checklist:

• Reviewers confirm new code works as expected.
• Tests are passing.
• Coverage does not decrease.
• Documentation is updated.
• Compatibility with Qibo modules (Please edit this section if the current pull request is not compatible with the following branches).
  • Qibo: master
  • Qibolab: main
  • Qibolab_platforms_qrc: main

[codecov]

Codecov Report

Merging #647 (c136ab4) into main (73ff1eb) will not change coverage.
Report is 4 commits behind head on main.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #647   +/-   ##
=======================================
  Coverage   96.03%   96.03%           
=======================================
  Files         106      106           
  Lines        7338     7338           
=======================================
  Hits         7047     7047           
  Misses        291      291           

Flag	Coverage Δ
unittests	96.03% <ø> (ø)

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Files	Coverage Δ
...cal/protocols/characterization/dispersive_shift.py	100.00% <ø> (ø)

[JavierSerranoGarcia]

Now works properly.