README: Fix configuration folder references
README: Fix configuration folder references

Change README.md, which refers to "project_settings" folder,
to the actual name "project_config".

Signed-off-by: Roger Pueyo Centelles <1753b7f5782ff92b25ed93145a118ac1fc93e83c@ac.upc.edu>

mcci-catena/arduino-lmic,mcci-catena/arduino-lmic,mcci-catena/arduino-lmic