Pins wrong on Tang Nano 20K

[Peter-van-Tol]

The pins on the connector J6 on the Tang Nano 20 K are incorrect (source litex_boards/platforms/sipeed_tang_nano_20k.py):

 _connectors = [
        ("J5", "- - - 76 80 42 41 56 54 51 48 55 49 86 79 - - 72 71 53 52"),
        ("J6", "- 73 74 85 77 15 16 27 28 25 26 29 30 31 17 20 19 18 - -")
]

should be:

 _connectors = [
        ("J5", "- - - 76 80 42 41 56 54 51 48 55 49 86 79 - - 72 71 53 52"),
        ("J6", "- 73 74 75 85 77 15 16 27 28 25 26 29 30 31 17 20 19 18 - -")
]

In the current version J5 has 21 entries (position 0 is not used) and J6 has 20 entries. Thus one pin is missing. According to the documentation pin 75 was the missing pin.