Fix incorrect sample code in documentation (bytecodealliance#6796)

Sample code provided in Readme of `wasmtime` crate specifies three
generic arguments for `get_typed_func` API, while it needs only two.
This fixes the sample code by removing the last generic argument.

crates/wasmtime/README.md

@@ -94,7 +94,7 @@ fn main() -> Result<()> {
     // this case we're using `4` for.
     let mut store = Store::new(&engine, 4);
     let instance = linker.instantiate(&mut store, &module)?;
-    let hello = instance.get_typed_func::<(), (), _>(&mut store, "hello")?;
+    let hello = instance.get_typed_func::<(), ()>(&mut store, "hello")?;
 
     // And finally we can call the wasm!
     hello.call(&mut store, ())?;