UG: nsteps.rst: suggest "mkdir -p" instead of plain "mkdir"

Use

    $ mkdir -p ~/bin

instead of

    $ mkdir ~/bin

to avoid errors in case ~/bin already exists.

Signed-off-by: Mario Domenech Goulart <mario@ossystems.com.br>
UG: nsteps.rst: add <selected machine> to the path to the .sdcard file

Also fix a small typo at the same line (s/select machine/selected machine/)

Signed-off-by: Mario Domenech Goulart <mario@ossystems.com.br>