fix: remove unused format verbs in error messages

[itchyny]

Description

This PR fixes unused format verbs in error messages.
I noticed that there is unused %v in the message while GitHub is down.
This should be occurred after 306b117.

{"level":"error","ts":1723678691.5177238,"msg":"error while updating the cache: %v\n","error":{"error":"request to...

Closes issue(s)

There is no reported issues.

Checklist

• I have tested this code
• I have added unit test to cover this code
• I have updated the documentation (README.md and /website/docs)
• I have followed the contributing guide

[netlify]

✅ Deploy Preview for go-feature-flag-doc-preview canceled.

Name	Link
🔨 Latest commit	6ace8d9
🔍 Latest deploy log	https://app.netlify.com/sites/go-feature-flag-doc-preview/deploys/66bda766b5910800076fd93d

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[thomaspoignant]

Good catch, I forgot to address this one when moving to slog.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 86.00%. Comparing base (2175f5d) to head (6ace8d9).
Report is 2 commits behind head on main.

Files	Patch %	Lines
feature_flag.go	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2223   +/-   ##
=======================================
  Coverage   86.00%   86.00%           
=======================================
  Files         102      102           
  Lines        3731     3731           
=======================================
  Hits         3209     3209           
  Misses        399      399           
  Partials      123      123           

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