docs: fix the table in pin.rst to match the diagram

[mattheww]

The pin table in pins.rst disagrees with the diagram:

• it says three of the edge connector pins are for the LED rows, rather than columns 7, 8, and 9;
• it has the three SPI pins in the opposite order

I think the diagram is right and the table is wrong:

• pinmap.csv says the three LED pins are internal GPIOs 10,11,12 which are definitely columns;
• spi.rst has sclk=pin13, mosi=pin15, miso=pin14 .

[dpgeorge]

Thanks for the contribution, you are correct!

The fix here also matches the code, source/microbit/microbitspi.cpp, and the display pin mappings in source/microbit/microbitdisplay.cpp