docs: minor aligment in `accessTokenType` attribute description

[PedroDiez]

Problem description
accessTokenType attribute description in CAMARA-API-Event-Subscription-and-Notification-Guide.md#21-instance-based-implicit-subscription Instance-based (Implicit) Subscription is not right. It must be aligned with the description indicated in Resource-based (Explicit) Subscription

Expected behavior

Align description to "Type of access token - MUST be set to bearer for now"

Alternative solution

N/A

Additional context

Editorial correction. No impact in functionality