vdir: link to RFCs explicitly

Author: WhyNotHugo

content/specs/vdir.md

@@ -41,11 +41,13 @@ collections contains items, which may be contacts or calendar components
 
 An item may only be one of the following two types:
 
-- A [vCard] file, in which case the file extension MUST be `.vcf`.
-- An [iCalendar] file, in which case the file extension MUST be `.ics`.
+- A vCard file, as defined in [RFC6450], in which case the file extension MUST
+  be `.vcf`.
+- An iCalendar file, as defined in [RFC5545], in which case the file extension
+  MUST be `.ics`.
 
-[vCard]: https://tools.ietf.org/html/rfc6350
-[iCalendar]: https://tools.ietf.org/html/rfc5545
+[RFC6450]: https://tools.ietf.org/html/rfc6350
+[RFC5545]: https://tools.ietf.org/html/rfc5545
 
 An item SHOULD contain a `UID` property as described by the vCard and iCalendar
 specifications. If it contains more than one `UID` property, the values of