curl.1: add missing exit-code

I noted a missing text for exit-code 89 in docs/curl.1

docs/curl.1

@@ -2067,6 +2067,8 @@ RTSP: mismatch of Session Identifiers
 unable to parse FTP file list
 .IP 88
 FTP chunk callback reported error
+.IP 89
+No connection available, the session will be queued
 .IP XX
 More error codes will appear here in future releases. The existing ones
 are meant to never change.