cmdline-opts/retry.d: mention response code 429 as well

Reported-by: Cherish98
Bug: https://curl.se/mail/archive-2020-12/0018.html

docs/cmdline-opts/retry.d

@@ -7,7 +7,7 @@ Category: curl
 If a transient error is returned when curl tries to perform a transfer, it
 will retry this number of times before giving up. Setting the number to 0
 makes curl do no retries (which is the default). Transient error means either:
-a timeout, an FTP 4xx response code or an HTTP 408 or 5xx response code.
+a timeout, an FTP 4xx response code or an HTTP 408, 429 or 5xx response code.
 
 When curl is about to retry a transfer, it will first wait one second and then
 for all forthcoming retries it will double the waiting time until it reaches