README.rst: Fix "Title underline too short" error

Fix "Title underline too short" error that is causing PyPI to render the description as plain text rather than RST.

This is an annoying aspect of PyPI -- see https://bitbucket.org/pypa/pypi/issue/214

Before:

```
❯ rst2html.py --halt=2 README.rst > /dev/null && echo "RST OK"
README.rst:2: (WARNING/2) Title underline too short.

Tornado Elasticsearch Client
===========================
Exiting due to level-2 (WARNING) system message.
```

After:

```
❯ rst2html.py --halt=2 README.rst > /dev/null && echo "RST OK"
RST OK
```