fix(monitor): revise support of Uptime Kuma

[favonia]

1. Fix the typo "Ok".
2. Document that the user should change the "Heartbeat Interval".
3. Always use the message "OK" instead of trying to send more information.

[codecov]

Codecov Report

Merging #605 (f75e5c5) into main (fb2e872) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #605   +/-   ##
=======================================
  Coverage   88.30%   88.30%           
=======================================
  Files          45       45           
  Lines        2300     2300           
=======================================
  Hits         2031     2031           
  Misses        259      259           
  Partials       10       10           

Flag	Coverage Δ
unittests	88.30% <100.00%> (ø)

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Files	Coverage Δ
internal/monitor/updatekuma.go	95.29% <100.00%> (ø)

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