Update KNX services.yaml (#26014)

Fixes wrong (outdated?) services.yaml for knx integration.

homeassistant/components/knx/services.yaml

@@ -1,5 +1,9 @@
-group_write:
-  description: Turn a light on.
+send:
+  description: "Send arbitrary data directly to the KNX bus."
   fields:
-    address: {description: Group address(es) to write to., example: 1/1/0}
-    data: {description: KNX data to send., example: 1}
+    address: 
+      description: "Group address(es) to write to."
+      example: "1/1/0"
+    payload: 
+      description: "Payload to send to the bus. Integers are treated as DPT 1/2/3 payloads. For DPTs > 6 bits send a list. Each value represents 1 octet (0-255). Pad with 0 to DPT byte length."
+      example: "[0, 4]"