Update link to SPDX specification on listing page

[steverydz]

Done

Updated the SPDX specification link

How to QA

• Go to https://snapcraft-io-4858.demos.haus/defold/listing (or any other snap listing page)
• Scroll down to the license field
• Click "Custom SPDX expression" radio button
• Check that the SPDX specification link works

Testing

• [ ] This PR has tests
• [x] No testing required (explain why): Link change

Issue / Card

Fixes #4856

[webteam-app]

Demo

Jenkins

demos.haus