NetworkPkg/HttpBootDxe: fix typo in DHCPv4 packet parsing
    
    The string "HTTPClient" has 10 non-NUL characters; the value 9 is a
    copy-paste leftover from "PXEClient". Check for all 10 characters in the
    vendor-class-identifier option when determining whether the DHCP offer is
    an HTTP offer.
    
    Cc: Jiaxin Wu <jiaxin.wu@intel.com>
    Cc: Siyuan Fu <siyuan.fu@intel.com>
    Contributed-under: TianoCore Contribution Agreement 1.1
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>