Editorial: Add event definition for 'close'

A reference to a 'close' event turned into a link back in d2bc4db but
there wasn't a local definition - it ended up linking into HTML! This
is now triggering a Bikeshed warning - yay!

Add a minimal local definition for the event type, like the others.

index.bs

@@ -500,11 +500,13 @@ running.
 A [=/connection=]'s [=get the parent=] algorithm returns
 null.
 
-A <dfn event>`versionchange`</dfn> will be fired at an open
+An event with type <dfn event>`versionchange`</dfn> will be fired at an open
 [=/connection=] if an attempt is made to upgrade or delete the
 [=database=]. This gives the [=/connection=] the opportunity to close
 to allow the upgrade or delete to proceed.
 
+An event with type <dfn event>`close`</dfn> will be fired at a [=/connection=] if the connection is [=/close a database connection|closed=] abormally.
+
 </div>