doc: edison: Update information about xFSTK

xFSTK sources got a new home under Edison Firmware Group on GitHub [1].
Update Intel Edison documentation accordingly.

While here, fix couple of typos.

[1]: https://github.com/edison-fw



Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Bin Meng <bmeng.cn@gmail.com>