Fix variable horizon link in 4_train_airl.ipynb

[Ivan-267]

Description

Fixes the variable horizon concept link from the tutorial for training AIRL.

Testing

I've tried the original link in master branch and the link from this PR branch.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (a8b079c) 95.69% compared to head (07b300b) 95.67%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #837      +/-   ##
==========================================
- Coverage   95.69%   95.67%   -0.03%     
==========================================
  Files         102      102              
  Lines        9645     9645              
==========================================
- Hits         9230     9228       -2     
- Misses        415      417       +2     

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