fix: Preferred() returns field with highest PREF
fix: Preferred() returns field with highest PREF

According to https://tools.ietf.org/html/rfc6350#section-5.3:
`Its value MUST be an integer between 1 and 100 that
   quantifies the level of preference.  Lower values correspond to a
   higher level of preference, with 1 being most preferred.`
emersion/go-vcard