fix: Correct verifiedpermissions_identity_source docs (#44834)

The identity_token_only object doesn't support a group_entity_type
attribute, but instead a principal_id_claim attribute.

AWS API ref: https://docs.aws.amazon.com/verifiedpermissions/latest/apireference/API_OpenIdConnectIdentityTokenConfiguration.html
Resource schema: https://github.com/hashicorp/terraform-provider-aws/blob/1ce57fb1864f09b65ee25ec911024a7e4010e34f/internal/service/verifiedpermissions/identity_source.go#L183-L192

Author: kwevers

website/docs/r/verifiedpermissions_identity_source.html.markdown

@@ -117,7 +117,7 @@ This resource supports the following arguments:
 #### Identity Token Only
 
 * `client_ids` - (Optional) The ID token audience, or client ID, claim values that you want to accept in your policy store from an OIDC identity provider.
-* `group_entity_type` - (Optional) The claim that determines the principal in OIDC access tokens.
+* `principal_id_claim` - (Optional) The claim that determines the principal in OIDC identity tokens.
 
 #### Open ID Group Configuration