serial_recovery: Reduce minimum number of members in packet

Was 3, but mcumgr sometimes sends 2 ("off" + "data", "len" is not needed
when "off" != 0). Reduce to 1 to avoid the problem, since the code
has other checks to catch this.

Signed-off-by: Øyvind Rønningstad <oyvind.ronningstad@nordicsemi.no>
diff --git a/boot/boot_serial/src/serial_recovery.cddl b/boot/boot_serial/src/serial_recovery.cddl
index 9aae63c..d45db08 100644
--- a/boot/boot_serial/src/serial_recovery.cddl
+++ b/boot/boot_serial/src/serial_recovery.cddl
@@ -11,5 +11,5 @@
 	("sha" => bstr)
 
 Upload = {
-	3**5members: Member
+	1**5members: Member
 }