docs: add target blank in example code

[afc163]

Checklist

• npm test passes
• tests are included
• documentation is changed or added
• commit message follows commit guidelines

Description of change

• any feature?
• close https://github.com/umijs/umi/ISSUE_URL

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View rendered docs/plugins/plugin-layout.zh-CN.md

[codecov]

Codecov Report

Merging #6087 (2c2c158) into master (bff5173) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #6087   +/-   ##
=======================================
  Coverage   82.94%   82.94%           
=======================================
  Files         158      158           
  Lines        3501     3501           
  Branches      941      943    +2     
=======================================
  Hits         2904     2904           
+ Misses        588      587    -1     
- Partials        9       10    +1     

Impacted Files	Coverage Δ
packages/core/src/Config/Config.ts	61.78% <0.00%> (ø)

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