RISC-V: Fix some typo in vector-iterators.md

This patch would like to fix some typo in vector-iterators.md, aka:

[-"vnx1DI")-]{+"vnx1di")+}
[-"vnx2SI")-]{+"vnx2si")+}
[-"vnx1SI")-]{+"vnx1si")+}

Signed-off-by: Pan Li <pan2.li@intel.com>

gcc/ChangeLog:

	* config/riscv/vector-iterators.md: Fix typo in mode attr.