Fix typo in documentation

[andrea-pasquale]

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 #523 (127a9a1) into main (aabc7f0) will decrease coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #523      +/-   ##
==========================================
- Coverage   96.51%   96.42%   -0.09%     
==========================================
  Files          66       66              
  Lines        4789     4789              
==========================================
- Hits         4622     4618       -4     
- Misses        167      171       +4     

Flag	Coverage Δ
unittests	96.42% <ø> (-0.09%)	⬇️

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