✏️ Fix link in fastapi-cli.md

[svlandeg]

As reported on Twitter: the link gave a 404 as the index.md page doesn't exist.

[tiangolo]

📝 Docs preview for commit 78b74c8 at: https://e57e68dc.fastapitiangolo.pages.dev

[svlandeg]

Link looks like it's fixed: https://e57e68dc.fastapitiangolo.pages.dev/fastapi-cli/

[tiangolo]

Thank you! 🍰

I tweaked it a bit to remove the ../ and point to the index.md so that MkDocs can detect it as an internal file and warn when it points to a different place (I should include those checks in the tests 🤔 ).

[tiangolo]

📝 Docs preview for commit e038e49 at: https://5d1fa315.fastapitiangolo.pages.dev