docs: use Returns: consistently

Instead of Return value:

modules/printbackends/papi/gtkprintbackendpapi.c

@@ -157,7 +157,7 @@ gtk_print_backend_papi_get_type (void)
  * implements the #GtkPrintBackend interface with direct access to
  * the filesystem using Unix/Linux API calls
  *
- * Return value: the new #GtkPrintBackendPapi object
+ * Returns: the new #GtkPrintBackendPapi object
  **/
 GtkPrintBackend *
 gtk_print_backend_papi_new (void)

modules/printbackends/papi/gtkprinterpapi.c

@@ -89,7 +89,7 @@ gtk_printer_papi_finalize (GObject *object)
  *
  * Creates a new #GtkPrinterPapi.
  *
- * Return value: a new #GtkPrinterPapi
+ * Returns: a new #GtkPrinterPapi
  *
  * Since: 2.10
  **/