fix(wren-ai-sevice): phony demo folder

[paopa]

I encountered an issue as follows when executing make demo command to start the demo-site streamlit app.

✦2 ➜ make demo  
make: `demo' is up to date.

and the root cause is having a same-name folder over there. thus, need to declare the demo in the .PHONY

See also:

• https://stackoverflow.com/questions/3931741/why-does-make-think-the-target-is-up-to-date
• https://stackoverflow.com/questions/2145590/what-is-the-purpose-of-phony-in-a-makefile