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>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Jiaxin Wu <jiaxin.wu@intel.com>