Corrected a typo in the README

When flashing with OpenOCD, you should use the .elf image, not the .bin.

README.md

 
 OpenOCD can also be used to flash the firmware.  For example:
 
-    $ openocd -f interface/stlink-v2.cfg -f target/stm32f0x.cfg -c "program build/pd-buddy-firmware.bin verify reset exit"
+    $ openocd -f interface/stlink-v2.cfg -f target/stm32f0x.cfg -c "program build/pd-buddy-firmware.elf verify reset exit"
 
 ## Usage