fix typo: forkTSCheker -> forkTSChecker

[sunnylqm]

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

[codecov]

Codecov Report

Merging #4590 into master will not change coverage.
The diff coverage is 50.00%.

@@           Coverage Diff           @@
##           master    #4590   +/-   ##
=======================================
  Coverage   82.74%   82.74%           
=======================================
  Files         144      144           
  Lines        2898     2898           
  Branches      721      716    -5     
=======================================
  Hits         2398     2398           
- Misses        491      492    +1     
+ Partials        9        8    -1     

Impacted Files	Coverage Δ
...set-built-in/src/plugins/features/forkTSChecker.ts	70.00% <50.00%> (ø)
packages/core/src/Config/Config.ts	61.15% <0.00%> (ø)

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[ycjcl868]

可能会有 breakChange，@sorrycc

[ycjcl868]

这个用的不多，可以及时先改，

[ycjcl868]

和 umi 3.2.0 一起发吧

[gao-sun]

I'm really shocked about this kind of typo and how did it pass code review, also the reply & typo in comment. :-(