fix bug in masked, multi-dimensional state space multi-threaded envs

[Graeme22]

This fixes what appears to be a copy/paste bug that affects multi-threaded envs where the state space of the wrapped environment has different dimensionality than the action space of that environment.

For an example environment that this breaks, see https://github.com/Graeme22/rlskedge

PR Checklist

• Update NEWS.md?
• Unit tests for all structs / functions?
• Integration and correctness tests using a simple env?
• PR Review?
• Add or update documentation?
• Write docstrings for new methods?

[codecov]

Codecov Report

Merging #900 (32604f2) into main (ea00fdf) will decrease coverage by 24.44%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #900       +/-   ##
==========================================
- Coverage   24.47%   0.04%   -24.44%     
==========================================
  Files         219     207       -12     
  Lines        7686    7361      -325     
==========================================
- Hits         1881       3     -1878     
- Misses       5805    7358     +1553     

Impacted Files	Coverage Δ
...src/algorithms/policy_gradient/multi_thread_env.jl	0.00% <0.00%> (ø)

... and 64 files with indirect coverage changes

[jeremiahpslewis]

@Graeme22 This looks like a good catch, thanks! Would you be able to contribute a test that would have caught the issue?