Update serverless-ai-hello-world.md

spin.toml was missing the ai-models declaration

Content must go through a pre-merge checklist.

Pre-Merge Content Checklist

This documentation has been checked to ensure that:

[ ] The title, template, and date are all set
[ ] Does this PR have a new menu item (anywhere in templates/*.hbs files) that points to a document .md that is set to publish in the future? If so please only publish the .md and .hbs changes in real-time (otherwise there will be a menu item pointing to a .md file that does not exist)
[ ] File does not use CRLF, but uses plain LF (hint: use cat -ve <filename> | grep $'\r' | wc -l and expect 0 as a result)
[ ] Has passed bart check
[ ] Has been manually tested by running in Spin/Bartholomew (hint: use PREVIEW_MODE=1 and run npm run styles to update styling)
[ ] Headings are using Title Case
[ ] Code blocks have the programming language set to properly highlight syntax and the proper copy directive
[ ] Have tested with npm run test and resolved all errors
[ ] Relates to an existing (potentially outdated) blog article? If so please add URL in blog to point to this content.