Changed documentation of channels.

[vodovozovaliza]

This PR replaces $\text{Tr}\rho$ with $\text{Tr}_q[\rho]$ in the documentation of DepolarizingChannel, ResetChannel and ThermalRelaxationChannel.

Checklist:

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

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (4d15903) 100.00% compared to head (6a0d7c1) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #808   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           97        97           
  Lines        12890     12890           
=========================================
  Hits         12890     12890           

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

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

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

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[AlejandroSopena]

Thanks @vodovozovaliza, now it is clearer.