mark issuance date as required

docs/build/identity.rs/1.0/docs/how-tos/verifiable-credentials/create.mdx

@@ -27,7 +27,7 @@ In the IOTA Identity Framework, you can create a Verifiable Credential with the
 - [**Subject**](https://www.w3.org/TR/vc-data-model/#credential-subject): The issuer's claims; a set of objects that contain one or more properties that are each related to a subject.
 - [**Issuer**](https://www.w3.org/TR/vc-data-model/#issuer): The identifier of the issuer, typically their [DID](../../explanations/decentralized-identifiers.mdx).
 - [**ID**](https://www.w3.org/TR/vc-data-model/#identifiers): Optional URI identifier for the credential.
-- [**Issuance Date**](https://www.w3.org/TR/vc-data-model/#issuance-date): Optional timestamp for expressing the date and time when a credential becomes valid.
+- [**Issuance Date**](https://www.w3.org/TR/vc-data-model/#issuance-date): Timestamp for expressing the date and time when a credential becomes valid.
 - [**Expiration Date**](https://www.w3.org/TR/vc-data-model/#expiration): Optional timestamp for expressing the date and time when a credential ceases to be valid.
 - [**Status**](https://www.w3.org/TR/vc-data-model/#status): Optional information used to determine the current status of a credential, i.e. whether or not it has been [revoked](./revocation.mdx).
 - [**Schema**](https://www.w3.org/TR/vc-data-model/#data-schemas): Optional list of objects specifying the schema that the data must conform to.