[docs]: fix custom theme prop example

[mzanelee]

Before submitting the PR, please make sure you do the following

• [x] Read the Contributing guide
• [x] Prefix your PR title with [@svelteui/core], [@svelteui/actions], [@svelteui/motion], [@svelteui/core], [core], or [docs].
• [ ] This message body should clearly illustrate what problems it solves.

Tests

• [ ] Run the tests with npm test and lint the project with yarn lint or just run yarn repo:prepush and check to see if it's passing.

[BeeMargarida]

Hi! The docs are correct, the createTheme comes from stitches, which returns an hashed class name (like it says in our docs). Stitches docs: https://stitches.dev/docs/api#createtheme