[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

llvm/docs/CodeGenerator.rst

@@ -2527,8 +2527,8 @@ When BPF_CLASS(code) == BPF_ALU or BPF_ALU64 or BPF_JMP,
 
 ::
 
-  BPF_X     0x0  use src_reg register as source operand
-  BPF_K     0x1  use 32 bit immediate as source operand
+  BPF_X     0x1  use src_reg register as source operand
+  BPF_K     0x0  use 32 bit immediate as source operand
 
 and four MSB bits store operation code