Fixing type annotation for EXPLAIN

[srittau]

Pull Request check-list

Please make sure to review and check all of these items:

• Does $ tox pass with this change (including linting)?
• Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• Is there an example added to the examples folder (if applicable)?
• Is the changes added to CHANGES file?

Item 1 was skipped, since I don't have an environment set up and setting it up seemed excessive for this simple fix. Items 3-6 don't apply to this PR.

Description of change

This fixes a simple typo, where a type annotation was used as a default value instead.

[codecov-commenter]

Codecov Report

Merging #1997 (81e9597) into master (9a1b00f) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1997      +/-   ##
==========================================
+ Coverage   92.94%   92.95%   +0.01%     
==========================================
  Files          78       78              
  Lines       16785    16785              
==========================================
+ Hits        15600    15602       +2     
+ Misses       1185     1183       -2     

Impacted Files	Coverage Δ
redis/commands/search/commands.py	89.24% <ø> (ø)
tests/test_cluster.py	98.29% <0.00%> (+0.12%)	⬆️

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[chayim]

Thanks @srittau great catch. Merging.