Correct a mistake in the docs for make_transport() (#159)

diff --git a/pyuavcan/VERSION b/pyuavcan/VERSION
index 26aaba0..6085e94 100644
--- a/pyuavcan/VERSION
+++ b/pyuavcan/VERSION
@@ -1 +1 @@
-1.2.0
+1.2.1
diff --git a/pyuavcan/application/_transport_factory.py b/pyuavcan/application/_transport_factory.py
index 7221224..ddca023 100644
--- a/pyuavcan/application/_transport_factory.py
+++ b/pyuavcan/application/_transport_factory.py
@@ -94,8 +94,9 @@
             E.g.: ``socketcan:vcan0``.
 
         * - ``uavcan.can.mtu``
-          - ``bit[1]``
+          - ``natural16[1]``
           - The MTU value to use with all constructed CAN transports.
+            Values other than 8 and 64 should not be used.
 
         * - ``uavcan.can.bitrate``
           - ``natural32[2]``