docs(paths-mapping): Fixed broken guide for paths mapping helper

[seongbin9786]

Summary

Hi! I'm a newcomer to ts-jest, had a problem setting up test environment for my project using this library.

TypeScript Alias using tsconfig.json didn't work in test files.

I was wondering for around 30 minutes and found the answer from the Issue's comment - which was at the bottom of the page. It seemed really hard to find the information.

This PR provides "helper" part of "Paths Mapping" documentation page the missing code.

As this Issue's comment states, the documentation should state as follows to make jest config work with aliases from tsconfig:

module.exports = {
  // [...]
  roots: ['<rootDir>'],
  modulePaths: [compilerOptions.baseUrl], // <-- This will be set to './src'
  moduleNameMapper: pathsToModuleNameMapper(compilerOptions.paths /*, { prefix: '<rootDir>/' } */),
}

FYI, the documentation was as follows(as you can see from commit):

  // [...]
  moduleNameMapper: pathsToModuleNameMapper(compilerOptions.paths /*, { prefix: '<rootDir>/' } */),
}

Test plan

Not needed.

Does this PR introduce a breaking change?

• Yes
• No

Other information

None.

[coveralls]

Pull Request Test Coverage Report for Build 3391529444

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.008%) to 95.861%

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Totals
Change from base Build 3351485920:	0.008%
Covered Lines:	4055
Relevant Lines:	4154

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 3410044521

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.03%) to 95.819%

---

Totals
Change from base Build 3351485920:	-0.03%
Covered Lines:	4055
Relevant Lines:	4154

---

💛 - Coveralls

[ahnpnl]

thanks!