doc: fix sphinx warning
doc: fix sphinx warning

Seen with recent version of sphinx:
WARNING: latex_preamble conflicts with latex_elements['preamble'], ignored.

Signed-off-by: Thomas Monjalon <5f50a84c1fa3bcff146405017f36aec1a10a9e38@monjalon.net>
Tested-by: Ferruh Yigit <ba8734c3c506d2e3557bd4ac8fdaccd25bdb9aa7@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk