Add Webhook annotation to list in spec

spec/src/main/asciidoc/microprofile-openapi-spec.asciidoc

@@ -263,6 +263,7 @@ The following annotations are found in the https://github.com/eclipse/microprofi
 | https://github.com/eclipse/microprofile-open-api/blob/main/api/src/main/java/org/eclipse/microprofile/openapi/annotations/servers/ServerVariable.java[@ServerVariable] | Represents a server variable for server URL template substitution.
 | https://github.com/eclipse/microprofile-open-api/blob/main/api/src/main/java/org/eclipse/microprofile/openapi/annotations/tags/Tag.java[@Tag] | Represents a tag for the API endpoint.
 | https://github.com/eclipse/microprofile-open-api/blob/main/api/src/main/java/org/eclipse/microprofile/openapi/annotations/tags/Tags.java[@Tags] | A container of multiple tags.
+| https://github.com/eclipse/microprofile-open-api/blob/main/api/src/main/java/org/eclipse/microprofile/openapi/annotations/callbacks/Webhook.java[@Webhook] | Represents a webhook that may be received. Similar to a callback but initiated using some mechanism other than a call to an operation (e.g. an admin UI).
 |===
 
 ===== Overrides