message: Fix example in runtime readme
diff: @@ -80,10 +80,10 @@ fn main() -> error::Result<()> {
  // We're not importing anything, so make an empty import object.
  let import_object = imports! {};
  - let mut instance = instantiate(WASM, &import_object)?;
  + let instance = instantiate(WASM, &import_object)?;
  let values = instance
  - .func("add_one")?
  + .dyn_func("add_one")?
  .call(&[Value::I32(42)])?;
  assert_eq!(values[0], Value::I32(43));
  