docs: minor fixes for code plugin howto

[ltalirz]

In particular, start following the exit code conventions.

[codecov]

Codecov Report

Merging #5014 (a729e42) into develop (7e67e1a) will increase coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #5014      +/-   ##
===========================================
+ Coverage    80.12%   80.13%   +0.01%     
===========================================
  Files          515      515              
  Lines        36694    36694              
===========================================
+ Hits         29398    29400       +2     
+ Misses        7296     7294       -2     

Flag	Coverage Δ
django	74.60% <ø> (+0.01%)	⬆️
sqlalchemy	73.52% <ø> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
aiida/transports/plugins/local.py	81.66% <0.00%> (+0.26%)	⬆️
aiida/transports/util.py	65.63% <0.00%> (+3.13%)	⬆️

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