docs: Fix dependency names

When gi-docgen added the "related" key, it also began checking
that the dependencies were actually dependencies otherwise they aren't
listed as dependencies.  The capitalization needs to match the
namespace.

docs/reference/gdk/gdk4.toml.in

@@ -18,12 +18,12 @@ search_index = true
   docs_url = "https://docs.gtk.org/gobject/"
 
   [dependencies."Gio-2.0"]
-  name = "GIO"
+  name = "Gio"
   description = "GObject Interfaces and Objects, Networking, IPC, and I/O"
   docs_url = "https://docs.gtk.org/gio/"
 
   [dependencies."cairo-1.0"]
-  name = "Cairo"
+  name = "cairo"
   description = "A 2D graphics library with support for multiple output devices"
   docs_url = "https://www.cairographics.org/manual/"
 

docs/reference/gsk/gsk4.toml.in

@@ -19,7 +19,7 @@ dependencies = ["Graphene-1.0", "Gdk-4.0"]
   docs_url = "https://ebassi.github.io/graphene/docs/"
 
   [dependencies."Gdk-4.0"]
-  name = "GDK"
+  name = "Gdk"
   description = "The GTK windowing system abstraction"
   docs_url = "https://docs.gtk.org/gdk4/"
 

docs/reference/gtk/gtk4.toml.in

@@ -14,12 +14,12 @@ search_index = true
 dependencies = ["Gdk-4.0", "Gsk-4.0"]
 
   [dependencies."Gdk-4.0"]
-  name = "GDK"
+  name = "Gdk"
   description = "The GTK windowing system abstraction"
   docs_url = "https://docs.gtk.org/gdk4/"
 
   [dependencies."Gsk-4.0"]
-  name = "GSK"
+  name = "Gsk"
   description = "The GTK rendering abstraction"
   docs_url = "https://docs.gtk.org/gsk4/"