Clarify TAP3 multi-role delegation client workflow

Clarify in step 4.5.2.1 of the client workflow that in a multi-role
delegation not each but rather a defined threshold of roles must
agree on the target hashes and lengths (see `min_roles_in_agreement`).

Author: lukpueh

tuf-spec.md

@@ -1259,9 +1259,9 @@ non-volatile storage as FILENAME.EXT.
     of appearance.
 
       * **4.5.2.1**. If the current delegation is a multi-role delegation,
-      recursively visit each role, and check that each has signed exactly the
-      same non-custom metadata (i.e., length and hashes) about the target (or
-      the lack of any such metadata).
+      recursively visit each role, and check that a defined threshold of
+      roles has signed exactly the same non-custom metadata (i.e., length and
+      hashes) about the target (or the lack of any such metadata).
 
       * **4.5.2.2**. If the current delegation is a terminating delegation,
       then jump to step 5.