Fix gradle dependency in download-and-installation.mdx

[ocadaruma]

Download and Installation guide has a typo in Gradle Groovy test dependencies. This PR fixes that

References

N/A

Submitter checklist

• The PR request is well described and justified, including the body and the references
• The PR title represents the desired changelog entry
• If the change against WireMock 2 functionality (incompatible with WireMock 3),
  it is submitted against the 2.x branch
• The repository's code style is followed (see the contributing guide)

Details: Contributor Guide

[tomakehurst]

Thanks!