fix: Correct search field in models.py

[fsbraun]

Description

This PR fixes #50.

Related resources

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Checklist

• I have opened this pull request against master
• I have added or modified the tests when changing logic
• I have followed the conventional commits guidelines to add meaningful information into the changelog
• I have read the contribution guidelines and I have joined #workgroup-pr-review on
  Slack to find a “pr review buddy” who is going to review my pull request.

[codecov]

Codecov Report

Merging #56 (680a5de) into master (f77faf9) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #56   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           14        14           
  Lines          160       160           
  Branches         6         6           
=========================================
  Hits           160       160           

Impacted Files	Coverage Δ
djangocms_file/models.py	100.00% <100.00%> (ø)

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