fix(monitor): correct printf format string

[favonia]

No description provided.

[codecov]

Codecov Report

Merging #265 (af0be8a) into main (c54237d) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #265   +/-   ##
=======================================
  Coverage   85.35%   85.35%           
=======================================
  Files          29       29           
  Lines        1946     1946           
=======================================
  Hits         1661     1661           
  Misses        274      274           
  Partials       11       11           

Flag	Coverage Δ
unittests	85.35% <100.00%> (ø)

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Impacted Files	Coverage Δ
internal/api/cloudflare.go	100.00% <100.00%> (ø)
internal/config/env.go	98.58% <100.00%> (ø)
internal/monitor/healthchecks.go	86.95% <100.00%> (ø)

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