Fix RST syntax error in README.

[JanCBrammer]

Pull Request check-list

Please make sure to review and check all of these items:

• Does $ tox pass with this change (including linting)?
• Does travis tests pass with this change (enable it first in your forked repo and wait for the travis build to finish)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?

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afterwards / while the PR is open.

Description of change

Added blank line to make code block visible in rendered RST.

[chayim]

Thanks @JanCBrammer for fixing this! Much appreciated.