The method takes an array of char, which actually is UTF-8, so make the docs say so.
Fixes #5543
/**
* gtk_css_provider_load_from_data:
* @css_provider: a `GtkCssProvider`
- * @data: (array length=length) (element-type guint8): CSS data loaded in memory
+ * @data: (array length=length) (element-type utf8): CSS data loaded in memory
* @length: the length of @data in bytes, or -1 for NUL terminated strings. If
* @length is not -1, the code will assume it is not NUL terminated and will
* potentially do a copy.