Fix wrong reference in `Retry` docstring

[PassionateBytes]

This change fixes a documentation issue that pushed me down a rabbit hole the other day. The documentation for the Retry exception mentions the max_retries configuration parameter, but that parameter does not exist. The actual name of the relevant parameter is max_tries instead.

[codecov-commenter]

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@@            Coverage Diff             @@
##             main     #507      +/-   ##
==========================================
- Coverage   96.27%   96.10%   -0.18%     
==========================================
  Files          11       11              
  Lines        1074     1078       +4     
  Branches      209      190      -19     
==========================================
+ Hits         1034     1036       +2     
- Misses         19       21       +2     
  Partials       21       21              

Files with missing lines	Coverage Δ
arq/worker.py	97.37% <ø> (+0.20%)	:arrow_up:

... and 1 file with indirect coverage changes

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