doc: fix body length in SMP UART transport

In the description of the partial-final frame of the SMP UART transport
specification was a typo in the maximum allowed raw body size. The size
must not be larger than MTU - 5 because there are 5 bytes occupied by
total length (2), frame termination (1) and crc (2).

Signed-off-by: Simon Moser <simon.github@mailbox.org>