Fix invalid long_description

[di]

This PR fixes invalid syntax in README.rst which will prevent upload to PyPI. Fixes #254.

Before:

$ python setup.py sdist bdist_wheel
...
$ twine check dist/*
Checking distribution dist/django_celery_beat-1.4.0-py2.py3-none-any.whl: warning: `long_description_content_type` missing.  defaulting to `text/x-rst`.
Failed
The project's long_description has invalid markup which will not be rendered on PyPI. The following syntax errors were detected:
line 218: Warning: Inline literal start-string without end-string.
Checking distribution dist/django-celery-beat-1.4.0.tar.gz: warning: `long_description_content_type` missing.  defaulting to `text/x-rst`.
Failed
The project's long_description has invalid markup which will not be rendered on PyPI. The following syntax errors were detected:
line 218: Warning: Inline literal start-string without end-string.
line 218: Warning: Inline literal start-string without end-string.

With this change:

$ python setup.py sdist bdist_wheel
...
$ twine check dist/*
Checking distribution dist/django_celery_beat-1.4.0-py2.py3-none-any.whl: warning: `long_description_content_type` missing.  defaulting to `text/x-rst`.
Passed
Checking distribution dist/django-celery-beat-1.4.0.tar.gz: warning: `long_description_content_type` missing.  defaulting to `text/x-rst`.
Passed

[auvipy]

I am surprised to see your PR!! thanks a lot :D