LENGTH isn't optional in timestamp.json for snapshot.json

In step 3 of the client application detailed workflow the spec states that the
client download "up to the number of bytes specified in the timestamp metadata
file" which suggests that LENGTH is not optional in the METAFILES entry for
snapshot.json in the timestamp.json file.

Signed-off-by: Joshua Lock <jlock@vmware.com>

Author: joshuagl

tuf-spec.md

@@ -979,9 +979,9 @@ repo](https://github.com/theupdateframework/specification/issues).
          "meta" : METAFILES
        }
 
-   METAFILES is the same is described for the snapshot.json file.  In the case
-   of the timestamp.json file, this MUST only include a description of the
-   snapshot.json file.
+   METAFILES is the same as is described for the snapshot.json file.  In the
+   case of the timestamp.json file, this MUST only include a description of the
+   snapshot.json file and the LENGTH of the snapshot.json file is REQUIRED.
 
    A signed timestamp.json example file: