[python docs] NDArray title section too short

[aaronmarkham]

Description

Sphinx builds of the Python API docs have the following warning(s):

/work/mxnet/docs/python_docs/python/build/api/ndarray/mxnet.ndarray.NDArray.rst:206: WARNING: Title underline too short.

In^place arithmetic operations
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

How to reproduce

You can run into the problem following these steps.

1. Run a "lite" binary build.

ci/build.py --docker-registry mxnetci --platform ubuntu_cpu_lite /work/runtime_functions.sh build_ubuntu_cpu_docs

2. Run the Python API docs build:

ci/build.py --docker-registry mxnetci --platform ubuntu_cpu_python /work/runtime_functions.sh build_python_docs

[mxnet-label-bot]

Hey, this is the MXNet Label Bot.
Thank you for submitting the issue! I will try and suggest some labels so that the appropriate MXNet community members can help resolve it.
Here are my recommended label(s): Doc