fix: error message typo

[jiangfucheng]

Type of change:

• Bugfix
• New feature provided
• Improve performance
• Backport patches
• Documentation
• Refactor
• Chore
• CI/CD or Tests

What this PR does / why we need it:

Pre-submission checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first

[Gallardot]

Good catch. Thank you for your contribution.

[codecov-commenter]

Codecov Report

Merging #1790 (9b78f4d) into master (3a8fdf6) will not change coverage.
The diff coverage is n/a.

❗ Current head 9b78f4d differs from pull request most recent head 792d9c0. Consider uploading reports for the commit 792d9c0 to get more accurate results

@@           Coverage Diff           @@
##           master    #1790   +/-   ##
=======================================
  Coverage   39.30%   39.30%           
=======================================
  Files          91       91           
  Lines        8073     8073           
=======================================
  Hits         3173     3173           
  Misses       4494     4494           
  Partials      406      406           

Impacted Files	Coverage Δ
pkg/apisix/cluster.go	28.14% <ø> (ø)

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