rpk/group/describe: fixed printing group instance id

Previously group instance id was printed as an address as pointer was
not dereferenced.

Signed-off-by: Michal Maslanka <michal@vectorized.io>

src/go/rpk/pkg/cli/cmd/group/describe.go

@@ -180,7 +180,7 @@ func printDescribedGroup(
 		headers = append(headers, "INSTANCE-ID")
 		orig := args
 		args = func(r *describeRow) []interface{} {
-			return append(orig(r), r.instanceID)
+			return append(orig(r), *r.instanceID)
 		}
 	}