Update release notes

Author: rly

docs/format/source/format_release_notes.rst

@@ -9,7 +9,7 @@ Release Notes
 Major changes
 ^^^^^^^^^^^^^
 - Added `BaseImage` and `ExternalImage` as new neurodata types. The first so both `Image` and `ExternalImage` can inherit from it. The second to store external images (#604)
-- Allowed `EventDetection` to have shape (num_events, 2) to store the channel index of the detected event. ()
+- Allowed `EventDetection` to have shape (num_events, 2) to store the channel index of the detected event. (#620)
 
 Minor changes
 ^^^^^^^^^^^^^