doc: remove repeated words in contribution guide
doc: remove repeated words in contribution guide

Signed-off-by: Yong Wang <d43454047dd416473224614364813c3ab52b395d@zte.com.cn>
Acked-by: John McNamara <af31595c36a4239b9676e9b3a777d3e2f15a6c4d@intel.com>

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