lib: correct link to the original source for div64_u64

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Jiri Kosina <trivial@kernel.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

lib/div64.c

@@ -87,7 +87,7 @@ EXPORT_SYMBOL(div_s64_rem);
  * by the book 'Hacker's Delight'.  The original source and full proof
  * can be found here and is available for use without restriction.
  *
- * 'http://www.hackersdelight.org/HDcode/newCode/divDouble.c'
+ * 'http://www.hackersdelight.org/HDcode/newCode/divDouble.c.txt'
  */
 #ifndef div64_u64
 u64 div64_u64(u64 dividend, u64 divisor)