Update installing download link to v2

[adamchainz]

Description

Update the link in the “installing” section to point to v2. The other installation methods link to version 2, but this one didn’t.

Corresponding issue: n/a

Testing

Opened link.

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  source changes)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue
• I ran the test suite locally (npm run test) and verified that it succeeded

[alexpetros]

Thank you!