fixup: add docs up map set operationC

Author: kaschula

src/pages/docs/liveobjects/rest-api-usage.mdx

@@ -556,6 +556,26 @@ The following example creates a new `LiveMap` instance and assigns it to the `po
 ```
 </Code>
 
+The following example shows how to update a key and value in a `LiveMap`:
+
+<Code>
+```shell
+  curl -X POST "https://main.realtime.ably.net/channels/my-channel/object" \
+    -u {{API_KEY}} \
+    -H "Content-Type: application/json" \
+    -d '{
+      "operation": "MAP_SET",
+      "path": "posts.post1",
+      "data": {
+        "key" : "title",
+        "value": {"string": "LiveObjects is still awesome"}
+      }
+    }'
+```
+</Code>
+
+Note in this example if the key `title` didn't exist it would have been created and set by this operation.
+
 When using the `path` specifier with a `COUNTER_CREATE` or `MAP_CREATE` operation, the server constructs *two* operations which are published as a [batch](#batch-operations):
 
 * A `MAP_CREATE` or `COUNTER_CREATE` operation used to create the new object