Fix a bug

[pilgrimygy]

Fix a bug in GridWorlds experiment.

[codecov-io]

Codecov Report

Merging #174 (235a050) into master (bc64e42) will not change coverage.
The diff coverage is 75.00%.

@@           Coverage Diff           @@
##           master     #174   +/-   ##
=======================================
  Coverage   60.32%   60.32%           
=======================================
  Files          67       67           
  Lines        2425     2425           
=======================================
  Hits         1463     1463           
  Misses        962      962           

Impacted Files	Coverage Δ
src/algorithms/policy_gradient/ppo.jl	85.29% <50.00%> (ø)
...periments/gridworlds/JuliaRL_BasicDQN_EmptyRoom.jl	92.30% <100.00%> (ø)
src/experiments/rl_envs/JuliaRL_SAC_Pendulum.jl	92.00% <100.00%> (ø)

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[findmyway]

Thanks!