docs: Don't use an add-symbol-file offset when describing gdb debugging

docs: Don't use an add-symbol-file offset when describing gdb debugging

Recent versions of gdb will only add an offset to the '.text' section
of object files via the add-symbol-file command.  The SeaBIOS build
doesn't place code directly in a '.text' section, so update the
documentation to obtain the same result via an 'objcopy' call instead.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>