spec: Remove unnecessary DPLLINK uses

Links from spec pages to spec pages never need to use absolute URLs,
and thus can be simple relative links.

This avoids breaking links due to the DPLLINK change in the following
commit.

Author: CyberShadow

spec/ddoc.dd

@@ -925,7 +925,7 @@ unittest
 ------
 
     $(P For more information please see the full section on
-    $(DPLLINK unittest.html#documented-unittests, documented unit tests).
+    $(LINK2 unittest.html#documented-unittests, documented unit tests).
 )
 
 $(H2 $(LNAME2 using_ddoc_for_other_documentation, Using Ddoc for other Documentation))

spec/interfaceToC.dd

@@ -56,7 +56,7 @@ int myDfunction(char[] s)
         or other such C
         $(LINK2 http://www.digitalmars.com/ctg/ctgLanguageImplementation.html#extended, extended type modifiers)
         in D. These are handled by
-        $(DPLLINK attribute.html#linkage, linkage attributes),
+        $(LINK2 attribute.html#linkage, linkage attributes),
         such as $(D extern (C)).)
 
         $(LI There is no volatile type modifier in D. To declare a C function that uses