Update onie-server.py

Fixed typo on line 94

Signed-off-by: Curt Brune <curt@cumulusnetworks.com>
x86 exec_installer: Fill serial number to HTTP headers

In powerpc platforms, the serial number is passed to kernel by
``bootargs`` from u-boot.  But it is not in x86 platforms.  It
makes wget send an empty serial number field as part of the HTTP
request headers during doing HTTP discover.

Signed-off-by: Curt Brune <curt@cumulusnetworks.com>