Fixes to toy model docstrings

[MichaelClerx]

• corrected inheritance
• replaced pints.ToyX with pints.toy.ToyX

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (34e9b31) to head (1f99582).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1727   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           97        97           
  Lines         9775      9776    +1     
=========================================
+ Hits          9775      9776    +1     

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