Insert missing space into 6.3 release notes

docs/releases/6.3.md

@@ -216,7 +216,7 @@ If access to JSON translation strings within JavaScript is needed, use `window.w
 
 The upgrade notification panel can still be removed with the [`WAGTAIL_ENABLE_UPDATE_CHECK = False`](update_notifications) setting.
 
-### `SiteSummaryPanel` is no longer removable with `construct_homepage_panels`hook
+### `SiteSummaryPanel` is no longer removable with `construct_homepage_panels` hook
 
 The summary items can still be removed with the [`construct_homepage_summary_items`](construct_homepage_summary_items) hook.