Fix syntax error in README

Author: Photonios

README.rst

@@ -20,7 +20,7 @@ With seamless we mean that any features we add will work truly seamlessly. You s
 Browse the documentation at: http://django-postgres-extra.readthedocs.io
 
 Major features
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 * Single query, concurrency safe upserts, or safely ignoring a duplicate insert.
 * Unique and not null constraints for `HStoreField`.
 * Signals for updates.