Fix documentation on CryptoEvent (#4628)

* Fix documentation on `CryptoEvent`

`CryptoApi` itself does not emit events (or at least, its public type
information does not allow you to listen for events emitted by CryptoApi).

* fix link

Author: richvdh

src/crypto-api/CryptoEvent.ts

@@ -15,7 +15,7 @@
  */
 
 /**
- * Events emitted by the {@link CryptoApi}
+ * Cryptography-related events emitted by the {@link matrix.MatrixClient}.
  */
 export enum CryptoEvent {
     /**