Fix typo in header.html docs

wagtail/admin/templates/wagtailadmin/shared/header.html

@@ -14,7 +14,7 @@
     - `merged` - if true, add the classname 'merged'
     - `action_url` - if present, display an 'action' button. This is the URL to be used as the link URL for the button
     - `action_text` - text for the 'action' button
-    - `action_icon` - icon for the 'action' button, default is 'icon-plus'
+    - `action_icon` - icon for the 'action' button, default is 'plus'
     - `extra_actions` - extra action buttons for the header. This is the HTML to be used for the extra buttons
     - `breadcrumb` - Custom breadcrumbs content as a variable, displayed in place of breadcrumb block before the header content
     - `header` - Replaces the header `h1` completely with custom variable content