fixed "unknown ca" error

README.md

 ### Generate CA key
 openssl genrsa -passout env:CA_PASSWD -aes256 -out zabbix-ca.key 4096
 ### Generate CA
-openssl req -x509 -new -key zabbix-ca.key -sha256 -days 36500 -out zabbix-ca.crt -passin env:CA_PASSWD -subj "/CN=zabbix-server"
+openssl req -x509 -new -key zabbix-ca.key -sha256 -days 36500 -out zabbix-ca.crt -passin env:CA_PASSWD -subj "/CN=zabbix-ca"
 
 ## Generate host cert