Fix readme for JS SDK (#46)

# Motivation

Readme for the JavaScript SDK mentioned some parameters as optional but
are mandatory.

# Changes

* Edit README.md in js-library.

# Tests

Not necessary.

# Todos

- [ ] Add entry to changelog (if necessary). Not necessary.

js-library/README.md

@@ -56,7 +56,7 @@ requestVerifiablePresentation({
   },
   issuerData: {
     origin: "<url of the origin>",
-    canisterId: "<[optional] canister id>",
+    canisterId: "<canister id>",
   },
   credentialData: {
     credentialSpec: {
@@ -67,7 +67,7 @@ requestVerifiablePresentation({
     },
     credentialSubject: "<user's principal>",
   },
-  identityProvider: "<[optional] url identity provider>",
+  identityProvider: "<url identity provider>",
   derivationOrigin: "<[optional] origin for delegated identity>",
   windowOpenerFeatures: "<[optional] window opener config string>",
 });