docs: Fix example in README.md (libp2p#54)

Currently, the example fails with
> Module '"@libp2p/peer-id"' declares 'PeerId' locally, but it is not exported.ts(2459)

Replaced with a working example.

Co-authored-by: Alex Potsides <[email protected]>

Author: laurentsenta

packages/libp2p-peer-id/README.md

@@ -38,11 +38,12 @@ A basic implementation of a peer id
 ## Example
 
 ```JavaScript
-import { PeerId } from '@libp2p/peer-id'
+import { peerIdFromString } from '@libp2p/peer-id'
 
-const id = new PeerId(...)
+const peer = peerIdFromString('k51qzi5uqu5dkwkqm42v9j9kqcam2jiuvloi16g72i4i4amoo2m8u3ol3mqu6s')
 
-console.log(id.toCid())
+console.log(peer.toCid()) // CID(bafzaa...)
+console.log(peer.toString()) // "12D3K..."
 ```
 
 ## API Docs