pidfd_send_signal(2): fix EINVAL wrt flags.

Fixes: 0343182 ("pidfd_send_signal(2): document new PIDFD_SIGNAL_* flags")
Signed-off-by: Kir Kolyshkin <[email protected]>

pidfd_send_signal.md

@@ -94,7 +94,7 @@ The calling process is not in a PID namespace from which it can send a
 signal to the target process.
 
 **EINVAL**
-*flags* is not 0.
+*flags* value is not valid.
 
 **EPERM**
 The calling process does not have permission to send the signal to the