Fix up description of knock_room_state field, which implied the required field was optional (matrix-org#1276)

Author: anoadragon453

changelogs/client_server/newsfragments/1276.clarification

@@ -0,0 +1 @@
+Fix various typos throughout the specification.

data/api/server-server/knocks.yaml

@@ -287,7 +287,7 @@ paths:
                 items:
                   $ref: "../../event-schemas/schema/core-event-schema/stripped_state.yaml"
                 description: |-
-                  An optional list of [stripped state events](/client-server-api/#stripped-state)
+                  A list of [stripped state events](/client-server-api/#stripped-state)
                   to help the initiator of the knock identify the room.
                 example:
                   $ref: "../../event-schemas/examples/knock_room_state.json"