correct config.toml.example

[IANTHEREAL]

What problem does this PR solve?

the configuration check-mb4-value-in-utf8 will never take effect in conf/config.toml.example

What is changed and how it works?

put check-mb4-value-in-utf8 into right section of config file according to the syntax of the toml configuration file

Check List

Tests

• Manual test (add detailed scripts or steps below)
  • create a config file, set check-mb4-value-in-utf8 = false
  • run tidb-server binary, check it takes effect

Related changes

• Need to cherry-pick to the release branch

[codecov-io]

Codecov Report

Merging #9313 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #9313      +/-   ##
==========================================
+ Coverage   67.19%   67.19%   +<.01%     
==========================================
  Files         371      371              
  Lines       77531    77531              
==========================================
+ Hits        52096    52099       +3     
+ Misses      20784    20777       -7     
- Partials     4651     4655       +4

Impacted Files	Coverage Δ
ddl/delete_range.go	75.13% <0%> (-2.65%)	⬇️
executor/join.go	77.86% <0%> (-1.05%)	⬇️
executor/distsql.go	73% <0%> (+0.46%)	⬆️
store/tikv/coprocessor.go	62.92% <0%> (+2.28%)	⬆️

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[tiancaiamao]

PTAL @winkyao

[zimulala]

LGTM

[zz-jason]

LGTM

[zz-jason]

/run-all-tests

[jackysp]

Why change it?

[IANTHEREAL]

also fixed in #9802