[devhelp] Book: update comment about DhLanguage

[Sébastien Wilmet <swilmet src gnome org>]

commit d02c0edf4dd601272d07661c6caaa05cec34a585
Author: Sébastien Wilmet <swilmet gnome org>
Date:   Fri Dec 29 19:42:14 2017 +0100

    Book: update comment about DhLanguage
    
    DhLanguage has been removed.

 src/dh-book.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/src/dh-book.c b/src/dh-book.c
index 1b45072..0513581 100644
--- a/src/dh-book.c
+++ b/src/dh-book.c
@@ -341,8 +341,8 @@ dh_book_new (GFile *index_file)
 
         /* Rewrite language, if any, including the prefix we want to use when
          * seeing it, to standarize how the language group is shown.
-         * FIXME: do this in DhLanguage instead (this may require to implement
-         * DhLanguage differently).
+         * FIXME: maybe instead of a string, have a DhLanguage object which
+         * canonicalizes the string.
          */
         dh_util_ascii_strtitle (language);
         priv->language = (language != NULL ?