spec: Fix open dfn tags

Signed-off-by: Pat Zielinski <git@patzielinski.com>

Author: patzielinski

tuf-spec.md

@@ -997,7 +997,7 @@ as is described for the <a>root.json</a> file.
     as key and <a>HASH</a> as defined for <a>METAFILES</a>.  For example:
     `{ "sha256": HASH, ... }`.
 
-  : <dfn>CUSTOM</a>
+  : <dfn>CUSTOM</dfn>
   ::
     An object.  If defined, the elements and values of the <a>CUSTOM</a> object
     will be made available to the client application.  The format of the
@@ -1265,7 +1265,7 @@ as is described for the <a>root.json</a> file.
     A string giving the location from which to retrieve metadata files.
     <a for="mirrors">METAPATH</a> will be a relative path to <a>URLBASE</a>.
 
-  : <dfn>TARGETSPATH</a>
+  : <dfn>TARGETSPATH</dfn>
   ::
     A string giving the location from which to retrieve target files.
     <a>TARGETSPATH</a> will be a relative path to <a>URLBASE</a>.