Fix allXY plot label and plot

[andrea-pasquale]

After merging #189 I changed the allXY routines because I thought that in the y axis we were supposed to plot the probability of the ground state (I got confused by the label in the plot). This PR fixes this issue.
Moreover, I changed the y label of the plot to "Expectation value of Z" to avoid future misunderstandings.

Checklist:

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

[codecov]

Codecov Report

Merging #221 (e58e3c2) into minor_fix_multiplex (73dcffe) will decrease coverage by 0.06%.
The diff coverage is 0.00%.

Additional details and impacted files

@@                   Coverage Diff                   @@
##           minor_fix_multiplex     #221      +/-   ##
=======================================================
- Coverage                37.13%   37.08%   -0.06%     
=======================================================
  Files                       19       19              
  Lines                     1349     1351       +2     
=======================================================
  Hits                       501      501              
- Misses                     848      850       +2     

Flag	Coverage Δ
unittests	37.08% <0.00%> (-0.06%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/qibocal/plots/allXY.py	11.90% <0.00%> (-0.30%)	⬇️

[stavros11]

I merged this in my branch and it works, thanks.