Fix config name from egg-Plugin to eggPlugin in plugin's doc

[hansenyang]

Checklist

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

Affected core subsystem(s)

Description of change

[codecov]

Codecov Report

Merging #1215 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1215   +/-   ##
=======================================
  Coverage   99.71%   99.71%           
=======================================
  Files          29       29           
  Lines         705      705           
=======================================
  Hits          703      703           
  Misses          2        2

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