fix(CONTRIBUTING.md): correct markdown link

[mgred]

Purpose

Small correction of a link to the Editor section of Prettier
[link] -> (link)

How to verify

1. Follow https://github.com/sinonjs/sinon/blob/master/CONTRIBUTING.md
2. Browse to Linting and style
3. See the second link to Prettier is broken

Checklist for author

• npm run lint passes
• References to standard library functions are cached.

[coveralls]

Pull Request Test Coverage Report for Build 2722

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 94.84%

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Totals
Change from base Build 2720:	0.0%
Covered Lines:	2059
Relevant Lines:	2131

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💛 - Coveralls

[mroderick]

Thanks!