[docbook-tc] Correction on DTD changes needed to implement Olinkproposal

Bob has pointed out to me that because Targetid attributes point
outside of the documents that contain them, Targetid needs to be
CDATA, not IDREF. So the corrected proposed ATTLIST for Olink will
look like this instead:

   <!ATTLIST olink
                    targetdoc        CDATA           #IMPLIED
                    targetid         CDATA           #IMPLIED
                    targetdocent     ENTITY          #IMPLIED
                    linkmode         IDREF           #IMPLIED
                    localinfo        CDATA           #IMPLIED
                    type             CDATA           #IMPLIED
                    %common.attrib;
                    %olink.role.attrib;
                    %local.olink.attrib;
   >