Update readme with client reference and challenge variable (passwordl…

…ess-id#17)

you're right, that wasn't clear. Thanks for the improvement

README.md

@@ -122,7 +122,8 @@ Example call:
 ```js
 import { client } from '@passwordless-id/webauthn' 
 
-const registration = await client.register("Arnaud", "a7c61ef9-dc23-4806-b486-2428938a547e", {
+const challenge = "a7c61ef9-dc23-4806-b486-2428938a547e"
+const registration = await client.register("Arnaud", challenge, {
  "authenticatorType": "auto",
  "userVerification": "required",
  "timeout": 60000,
@@ -243,7 +244,8 @@ Example call:
 ```js
 import { client } from 'webauthn'
 
-const authentication = await webauthn.authenticate(["3924HhJdJMy_svnUowT8eoXrOOO6NLP8SK85q2RPxdU"], "56535b13-5d93-4194-a282-f234c1c24500", {
+const challenge = "56535b13-5d93-4194-a282-f234c1c24500"
+const authentication = await client.authenticate(["3924HhJdJMy_svnUowT8eoXrOOO6NLP8SK85q2RPxdU"], challenge, {
  "authenticatorType": "auto",
  "userVerification": "required",
  "timeout": 60000