samples: debug/gdbstub: update instructions to run

This corrects the directory pointing to sample in the README
file. Also changes the TCP port number for the second serial
port exposing the GDB stub. This allows QEMU itself to expose
GDB interface through port 1234, and Zephyr's GDB stub at
port 5678.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>