Fix formatting error of readme (facebookresearch#719)

Summary:
Pull Request resolved: facebookresearch#719

Readme format broke due to extra set of code blocks.

Reviewed By: adellari

Differential Revision: D72402739

fbshipit-source-id: 614522ea792f78e8d1ea52fa8a8f0a7e541d0467

Author: JasonKChow

Readme.md

@@ -33,7 +33,6 @@ You can also run the server in read-only mode, which creates a temporary copy of
 ```
 aepsych_server --port 5555 --ip 0.0.0.0 --db mydatabase.db --read-only
 ```
-```
 
 The server accepts messages over a unix socket, and
 all messages are formatted using [JSON](https://www.json.org/json-en.html). All messages