api docs: replace subject with topic in Curl example.

`subject` has been a deprecated alias since zulip 2.0.0.

It seems to be the only hardcoded example according to c4d805a82c

templates/zerver/api/send-message.md

@@ -18,7 +18,7 @@ curl -X POST {{ api_url }}/v1/messages \
     -u BOT_EMAIL_ADDRESS:BOT_API_KEY \
     --data-urlencode type=stream \
     --data-urlencode to=Denmark \
-    --data-urlencode subject=Castle \
+    --data-urlencode topic=Castle \
     --data-urlencode 'content=I come not, friends, to steal away your hearts.'
 
 # For private messages