Fix typo in SSL_CTX_set_msg_callback docs

Change `SSL_get_msg_callback_arg` to `SSL_set_msg_callback_arg`
Signed-off-by: NRich Salz <rsalz@akamai.com>
Reviewed-by: NTim Hudson <tjh@openssl.org>

doc/ssl/SSL_CTX_set_msg_callback.pod

@@ -2,7 +2,7 @@
 
 =head1 NAME
 
-SSL_CTX_set_msg_callback, SSL_CTX_set_msg_callback_arg, SSL_set_msg_callback, SSL_get_msg_callback_arg - install callback for observing protocol messages
+SSL_CTX_set_msg_callback, SSL_CTX_set_msg_callback_arg, SSL_set_msg_callback, SSL_set_msg_callback_arg - install callback for observing protocol messages
 
 =head1 SYNOPSIS