docs: fix better-auth tutorial

[typed-sigterm]

The original usage is incorrect.

If we pass the prefix to .mount, paths like /auth/sign-in/social would throw 404.

[gokaybiz]

It looks like you stripped out the sample snippet part completely? I mean if we shouldn't use mount chain then what should we prefer?

[typed-sigterm]

I mean to use .mount as in the previous section (only pass 1 parameter) and set the prefix in better-auth.