Fix store example
//! `wasm-unknown-unknown` an indexeddb store would be needed
//!
//! ```
+//! # use std::sync::Arc;
//! # use matrix_sdk_crypto::{
//! # OlmMachine,
//! # store::MemoryStore,
//! # use ruma::{device_id, user_id};
//! # let user_id = user_id!("@example:localhost");
//! # let device_id = device_id!("TEST");
-//! let store = Box::new(MemoryStore::new());
+//! let store = Arc::new(MemoryStore::new());
//!
//! let machine = OlmMachine::with_store(user_id, device_id, store);
//! ```
