rustdoc: fix broken tooltip CSS
text `#ffffff` on background `#fdffd3` fails the [WCAG color contrast checker], and seems like a mistake in https://github.com/rust-lang/rust/commit/
16b55903ee16503e7026677c169727d1907704c2.
Making the cursor a pointer is misleading, since clicking it doesn't do anything.
[WCAG color contrast checker]: https://accessibleweb.com/color-contrast-checker/