fix(docs): AXES_CLIENT_IP_CALLABLE documentation

[OscarVanL]

What does this PR do?

Previously, the readme suggested the AXES_CLIENT_IP_CALLABLE function should have two arguments, but actually it only takes one argument.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).

[aleksihakli]

Thank you @OscarVanL!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.54%. Comparing base (3fa7fce) to head (6ba4090).
Report is 14 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1252      +/-   ##
==========================================
+ Coverage   93.53%   93.54%   +0.01%     
==========================================
  Files          36       36              
  Lines        1175     1177       +2     
  Branches      216      216              
==========================================
+ Hits         1099     1101       +2     
  Misses         57       57              
  Partials       19       19              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.