Fix functions.rst for JDK 21

[penghuo]

Description

Fix functions.rst failure for JDK 21. https://github.com/opensearch-project/sql/actions/runs/7794417851/job/21255718994?pr=2505

Issues Resolved

n/a

Check List

• New functionality includes testing.
  • All tests pass, including unit test, integration test and doctest
• New functionality has been documented.
  • New functionality has javadoc added
  • New functionality has user manual doc added
• Commits are signed per the DCO using --signoff

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (1f4390e) 95.41% compared to head (b205d23) 95.41%.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #2506   +/-   ##
=========================================
  Coverage     95.41%   95.41%           
  Complexity     5026     5026           
=========================================
  Files           483      483           
  Lines         14005    14005           
  Branches        944      944           
=========================================
  Hits          13363    13363           
  Misses          621      621           
  Partials         21       21           

Flag	Coverage Δ
sql-engine	95.41% <ø> (ø)

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