Improve clone command in README

Without the explicit dir at the end the repository will be cloned to "code".

Change-Id: Icd8b55b4ba74f23b214c3844e2fb785377768119
Signed-off-by: Freddie Chopin <freddie.chopin@gmail.com>
Reviewed-on: http://openocd.zylin.com/1349
Reviewed-by: Øyvind Harboe <oyvindharboe@gmail.com>
Tested-by: jenkins