[glib] giomodule: Fix a copy/paste error in some internal documentation

[Philip Withnall <pwithnall src gnome org>]

commit 891026d9fc0a7270c2c66df50d65bc3f328da35e
Author: Philip Withnall <withnall endlessm com>
Date:   Wed Mar 21 15:32:13 2018 +0000

    giomodule: Fix a copy/paste error in some internal documentation
    
    Signed-off-by: Philip Withnall <withnall endlessm com>
    Reviewed-by: nobody

 gio/giomodule.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/gio/giomodule.c b/gio/giomodule.c
index 1ead84cf8..72a12ea45 100644
--- a/gio/giomodule.c
+++ b/gio/giomodule.c
@@ -722,8 +722,8 @@ try_class (GIOExtension *extension,
  * The result is cached after it is generated the first time, and
  * the function is thread-safe.
  *
- * Returns: (transfer none): an object implementing
- *     @extension_point, or %NULL if there are no usable
+ * Returns: (transfer none): the type to instantiate to implement
+ *     @extension_point, or %G_TYPE_INVALID if there are no usable
  *     implementations.
  */
 GType