Correction to Guided_Tutorial_PSK_Demodulation

[Barry Duggan]

Refs:
- 
https://wiki.gnuradio.org/index.php/Guided_Tutorial_PSK_Demodulation#Decoding
- https://github.com/gnuradio/gnuradio/issues/3599

I have found, corrected, and tested the problems with mpsk_stage6.grc
The order of blocks should be Constellation_Decoder, 
Differential_Decoder, and Map.
The receive Map should be [0,1,2,3].
The corrected flowgraph is in https://pastebin.com/2mRCzmPF
Once you go to the pastebin link, you need to click on the 'raw' icon to 
get the unformatted text.

I have tested with 3.8.1.0 and 3.9.0.0-git. The author of issue 3599 
tested with 3.7.13.5. If any of you can test it for me before I write a 
PR, I would appreciate it!

Thanks :)
--
Barry Duggan KV4FV