RFC14,25: fix reference to the "Reserved Resource Types"

Problem: RFC25 references a section that only exists in the canonical
jobspec specification (RFC14).

Solution: make an explicit label for the section and reference it from
RFC25 with `:ref:`.

Author: SteVwonder

spec_14.rst

@@ -220,6 +220,8 @@ following keys
    matching resource identifiers.
 
 
+.. _rfc14-reserved-resource-types:
+
 Reserved Resource Types
 ^^^^^^^^^^^^^^^^^^^^^^^
 

spec_25.rst

@@ -100,8 +100,7 @@ following:
 **type**
    The ``type`` key for a resource SHALL indicate the type of resource to be
    matched. In V1, only four resource types are valid: [``node``, ``slot``, ``core``,
-   and ``gpu``]. ``slot`` types are described in the **Reserved Resource Types** section
-   below.
+   and ``gpu``]. ``slot`` types are described in the :ref:`rfc14-reserved-resource-types`.
 
 **count**
    The ``count`` key SHALL indicate the desired number of