[RTM] Removed an occurrence of nb_q

[pariterre]

This change is 

[codecov]

Codecov Report

Base: 81.49% // Head: 81.49% // Decreases project coverage by -0.01% ⚠️

Coverage data is based on head (e3ef0b5) compared to base (258a307).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #612      +/-   ##
==========================================
- Coverage   81.49%   81.49%   -0.01%     
==========================================
  Files         106      106              
  Lines       11776    11775       -1     
==========================================
- Hits         9597     9596       -1     
  Misses       2179     2179              

Impacted Files	Coverage Δ
...xamples/getting_started/example_external_forces.py	87.87% <100.00%> (ø)
bioptim/optimization/optimal_control_program.py	87.13% <100.00%> (-0.03%)	⬇️

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[Ipuch]

@pariterre There is a new version of black :)