Fix several minor issues with C++ custom ops (`adding_an_op.md`) documentation (#8546)

This fix tries to address several issues related to C++ custom ops
(`adding_an_op.md`) documentation:
1. The `tf.load_op_library('zero_out.so')` should use full path or relative path (`./`)
   (This issue has been raised in #4727 as well).
2. The mentioning of `zero_out_op_1.py` and `tf.load_op_library('zero_out_op_kernel_1.so')`
   is really misleading. While `zero_out_op_1.py` is located in
   `tensorflow/examples/adding_an_op/`, the documentation (`adding_an_op.md`) does not
   really use `zero_out_op_1.py`. It is better to use the `zero_out.cc` that
   is presented in the doc.

This fix changes `tf.load_op_library('zero_out.so')` to `tf.load_op_library('./zero_out.so')`
and updates a few places so that following the documentation could have the expected result.

This fix fixes #4727.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>