need `-f` flag when `kinit` to request a forward-able tickets

ref. https://web.mit.edu/kerberos/krb5-1.12/doc/user/tkt_mgmt.html

docs/docs/kerberos-access.md

@@ -53,7 +53,7 @@ from a Java JRE, these often have problems such as echoing the
 password).
 
 ```bash
-$ kinit afsusername@CERN.CH
+$ kinit -f afsusername@CERN.CH
 afsusername@CERN.CH's Password: <enter your password>
 $
 ```