Fix: add missing main landmarks (a11y)

[EmmaDawsonDev]

Description

The download page was missing a main tag so I changed the wrapping div tag to a main tag to be semantically correct and work better for users of assistive tech.

Related Issues

No related issue

Check List

• I have read the Contributing Guidelines and made commit messages that follow the guideline.
• I have run npm run lint:js -- --fix and/or npm run lint:md -- --fix for my JavaScript and/or Markdown changes.
  • This is important as most of the cases your code changes might not be correctly linted
• I have run npm run test to check if all tests are passing, and/or npm run test -- -u to update snapshots if I created and/or updated React Components.
• I have checked that the build works locally and that npm run build work fine.
• I've covered new added functionality with unit tests if necessary.

[ovflowd]

Nice little addition! Thank you!