doc: samsung: Fix PXE doc for E850-96 board

doc: samsung: Fix PXE doc for E850-96 board

As stated in PXELINUX doc [1], the PXE configuration file has to be in
the format of "01-MAC-address" for Ethernet connections:

    The hardware type (using its ARP "htype" code) and address, all in
    lowercase hexadecimal with dash separators. For example, for an
    Ethernet (i.e. ARP hardware type "1") with address
    "88:99:AA:BB:CC:DD", it would search for the filename
    "01-88-99-aa-bb-cc-dd".

Indeed, PXE implementation in U-Boot looks for files like that, as can
be seen from this call chain:

    format_mac_pxe()
    pxe_mac_path()
    pxe_get()
    extlinux_pxe_read_bootflow()

Mention the fact that PXE expects the configuration file to be prepended
with "01" in the PXE section of E850-96 documentation. While at it, fix
some other minor issues in PXE section.

[1] https://wiki.syslinux.org/wiki/index.php?title=PXELINUX

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>