glib: Fix introspection for poppler_document_new_from_data

The API takes binary data, not an UTF-8 string.
diff --git a/glib/ b/glib/
index 78a57b1..cd7e8eb 100644
--- a/glib/
+++ b/glib/
@@ -223,7 +223,7 @@
  * poppler_document_new_from_data:
- * @data: the pdf data contained in a char array
+ * @data: (array length=length) (element-type guint8): the pdf data
  * @length: the length of #data
  * @password: (allow-none): password to unlock the file with, or %NULL
  * @error: (allow-none): Return location for an error, or %NULL