Consistently use TODO for to-dos.

lib/zip_string.c

diff --git a/lib/zip_string.c b/lib/zip_string.c
index a2d5eb5..4e8709b 100644
--- a/lib/zip_string.c
+++ b/lib/zip_string.c
@@ -64,7 +64,7 @@
     if (a->length != b->length)
 	return 0;
 
-    /* XXX: encoding */
+    /* TODO: encoding */
 
     return (memcmp(a->raw, b->raw, a->length) == 0);
 }