gallium/docs: Fix the math formula of U2I64

before:
  dst.xy = (uint64_t) src0.x
  dst.zw = (uint64_t) src0.y

after:
  dst.xy = (int64_t) src0.x
  dst.zw = (int64_t) src0.y

Signed-off-by: Mun Gwan-gyeong <elongbug@gmail.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
1 file changed