Fix type of dotfiles property

[MSE99]

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

Note

Fixes #165

[zekth]

can you add test for dotfile? thanks

[mcollina]

See https://github.com/fastify/fastify-static/blob/master/test/types/index.ts for the test

[MSE99]

The CI failed, it's because the tests failed due to the fact i tried to dynamically add an .example file for each test (it worked and the tests passed on my local machine) i did this because i didn't want to conflict with the last pull request since adding a file to the static directory will require editing other test, since this branch is behind master can i merge the changes from master into it and just manually add the .example file into the static directory ?

[mcollina]

lgtm

[zekth]

lgtm. Thanks for adding the tests