cluster: add tikv_worker_servers to topology.example.yaml

[JaySon-Huang]

What problem does this PR solve?

close #xxx

What is changed and how it works?

add tikv_worker_servers to topology.example.yaml

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

Release notes:

NONE

[ti-chi-bot]

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 42.30%. Comparing base (0ca2354) to head (42cb616).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2718      +/-   ##
==========================================
- Coverage   42.32%   42.30%   -0.02%     
==========================================
  Files         426      426              
  Lines       47330    47330              
==========================================
- Hits        20030    20021       -9     
- Misses      24605    24611       +6     
- Partials     2695     2698       +3     

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