docs: separate public/private principal examples

Author: alanshaw

README.md

@@ -21,7 +21,9 @@ See examples in [principals_test.go](./examples/principals_test.go)
 ```go
 principal, err := did.Parse("did:key:z6MkfBSb2hC6g3UGnqNmWfmGvPdfMorBpT2osm9bk9b4Cyqu")
 fmt.Println("DID:", principal.DID())
+```
 
+```go
 // generate a new ed25519 signer (it is also a principal - has a DID() method)
 signerPrincipal, err := ed25519.Generate()
 fmt.Println("DID:", signer.DID())