action_dim -> action_shape

[Squeemos]

Noticed that in IQN and FQF there were some mismatches in the docstrings. Figured I would make a pull request to make it match. Causes an exception if using kwargs over args.

• I have marked all applicable categories:
  • exception-raising fix
  • algorithm implementation fix
  • documentation modification
  • new feature
• I have reformatted the code using make format (required)
• I have checked the code using make commit-checks (required)
• If applicable, I have mentioned the relevant/related issue(s)
• If applicable, I have listed every items in this Pull Request below

[codecov-commenter]

Codecov Report

Merging #602 (d1b54b7) into master (57ecebd) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #602   +/-   ##
=======================================
  Coverage   93.54%   93.54%           
=======================================
  Files          67       67           
  Lines        4433     4433           
=======================================
  Hits         4147     4147           
  Misses        286      286           

Flag	Coverage Δ
unittests	93.54% <ø> (ø)

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Impacted Files	Coverage Δ
tianshou/utils/net/discrete.py	100.00% <ø> (ø)

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[nuance1979]

LGTM