krb5_set_default_realm.3 man page update

Since the memory is allocated inside the Kerberos library, it
should be freed by code inside the same library.  free, as
previously recommended, therefore doesn't seem appropriate.
Instead, recommend krb5_xfree, which exists for this purpose.
krb5_set_default_realm.3 man page update

Change-Id: I11d119edf03148cbdc654480c72ddffb540084ec