[Docs][CodeGenerator][eBPF] Correct the values for BPF_X and BPF_K

Fix the values of BPF_X and BPF_K according to BPFInstrFormats.td:
"
def BPF_K : BPFSrcType<0x0>;
def BPF_X : BPFSrcType<0x1>;
"

The right value for BPF_X is 0x1, and the right value for BPF_K is 0x0.

Signed-off-by: Wang YanQing <udknight@gmail.com>

Differential Revision: https://reviews.llvm.org/D61512

llvm-svn: 359904