fix: single-node cluster example in Istiod HA docs

[chinaran]

What type of PR is this?

• Enhancement / New Feature
• Bug Fix
• Refactor
• Optimization
• Test
• Documentation Update

What this PR does / why we need it:

Update the Considerations for Single-Node Clusters example in docs/general/istiod-ha.adoc to place global.defaultPodDisruptionBudget under spec.values, matching the API reference and the correct Istio resource structure.

[istio-testing]

Hi @chinaran. Thanks for your PR.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.63%. Comparing base (a39d566) to head (360fd9c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1915      +/-   ##
==========================================
+ Coverage   80.29%   80.63%   +0.34%     
==========================================
  Files          51       51              
  Lines        2598     2598              
==========================================
+ Hits         2086     2095       +9     
+ Misses        387      381       -6     
+ Partials      125      122       -3     

Flag	Coverage Δ
integration-tests	71.34% <ø> (+0.39%)	⬆️
unit-tests	52.30% <ø> (ø)

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[FilipB]

Thank you!