update bounds keyword docs for CMAES

[scarrazza]

Closes #771.

Checklist:

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

[scarlehoff]

For me it is ok. An error would prompt me to look for the cma documentation (although is not super easy to find) so it fixes my problem.

But maybe it would be good to add an example to the qibo documentation precisely because the cma documentation is not that easy to find…

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (87ed54f) 100.00% compared to head (2eb6a61) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #861   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           48        48           
  Lines         6370      6370           
=========================================
  Hits          6370      6370           

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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

Impacted Files	Coverage Δ
src/qibo/optimizers.py	100.00% <100.00%> (ø)

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