Fix plot_perturbation to ensure nbl is not plotted

[cako]

Description

plot_perturbation has a small bug where the nbl is not properly addressed when plotting. This PR addressed that by applying the same slicing as plot_velocity.

[EdCaunt]

Minor comment, but generally a useful fix. Thanks!

[EdCaunt]

I'm happy with this once CI is green

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.96%. Comparing base (828497e) to head (78020ca).

Files with missing lines	Patch %	Lines
examples/seismic/plotting.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2797      +/-   ##
==========================================
- Coverage   78.97%   78.96%   -0.01%     
==========================================
  Files         248      248              
  Lines       50936    50938       +2     
  Branches     4404     4404              
==========================================
- Hits        40226    40224       -2     
- Misses       9909     9912       +3     
- Partials      801      802       +1     

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[cako]

@EdCaunt seems like the linting is failing elsewhere:

warning: Invalid rule code provided to `# noqa` at devito/mpi/distributed.py:99: F824

[EdCaunt]

@cako can you check this is rebased on latest main? If not, then rebase this branch and force push it.
Also the linting is failing with trailing whitespace on your tweaks

warning: Invalid rule code provided to `# noqa` at devito/mpi/distributed.py:99: F824
Error: examples/seismic/plotting.py:34:80: W291 Trailing whitespace
Error: Process completed with exit code 1.

Note that the first line is only a warning.

[cako]

done