Make delegated targets freeze attack consistent

Other freeze attack checks state that the listed timestamp
must be higher than the fixed update start time. Use the
same wording in the delegated targets freeze attack check.

Signed-off-by: Marina Moore <mnm678@gmail.com>

Author: mnm678

tuf-spec.md

@@ -1520,9 +1520,9 @@ it in the next step.
        DELEGATOR metadata file.  If the new DELEGATEE metadata file is not signed
        as required, abort the update cycle, and report the failure.
 
-    6. **Check for a freeze attack.** The latest known time
-       should be lower than the expiration timestamp in the new DELEGATEE
-       metadata file. If so, the new DELEGATEE file becomes the trusted DELEGATEE
+    6. **Check for a freeze attack.** The expiration timestamp in new
+       DELEGATEE metadata file MUST be higher than the fixed update start time.
+       If so, the new DELEGATEE file becomes the trusted DELEGATEE
        file. If the new DELEGATEE metadata file is expired, abort the update
        cycle, and report the potential freeze attack.