The source project of this merge request has been removed.
doc: Fix there/their typo
Common mistake, but mistake nonetheless.
(Somewhat related: On the web version, https://developer.gnome.org/gtk3/stable/gtk3-Standard-Enumerations.html#GtkShadowType, there's an extra space, "(GTK_SHADOW_NONE )". It's not in the repo, and near-impossible to notice without copypasting or knowing it's there, so I suspect it's not worth tracking down.)