docs: update version to 0.24.0

Signed-off-by: Wei Zhang <kweizh@gmail.com>

docs.md

@@ -72,7 +72,7 @@ Be sure to add the following section to your Cargo.toml:
 
 ```
 [build-dependencies]
-cbindgen = "0.20.0"
+cbindgen = "0.24.0"
 ```
 
 If you'd like to use a `build.rs` script with a `cbindgen.toml`, consider using [`cbindgen::generate()`](https://docs.rs/cbindgen/*/cbindgen/fn.generate.html) instead.