doc: debug plant parameter vector in MTK example of the manual

[franckgaga]

Following discussion in #340, I'm sticking to the unofficial and limited-in-scope solution:

ic = initial_conditions(io_sys)
p_map = Dict(sym => ic[sym] for sym in p_sym)
p = ModelingToolkit.varmap_to_vars(p_map, p_sym)

to fetch the parameter vector. It gives a parameter vector p with the same length as p_sym, which is clearer for the user, IMO.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.57%. Comparing base (53c94fa) to head (841cbf2).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

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##             main     #342   +/-   ##
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  Coverage   98.57%   98.57%           
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  Files          27       27           
  Lines        5190     5190           
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  Hits         5116     5116           
  Misses         74       74           

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