doc: update the recommended placeholder

Currently recommended `expect![[]]` is broken rust-analyzer#37, and rust-analyzer#38 is blocked for long time, so maybe we can update the recommendation to a safer option instead.

Signed-off-by: xxchan <[email protected]>

src/lib.rs

@@ -113,7 +113,7 @@
 //! # fn check(_: i32, _: Expect) {}
 //! #[test]
 //! fn test_division() {
-//!     check(92 / 2, expect![[]])
+//!     check(92 / 2, expect![[""]])
 //! }
 //! ```
 //!