DOC: hashing: update link to hashing functions

Bret Mulvey, the author of the article cited in this pulication
has migrated his work to papa.bretmulvey.com. I was able to
view an archival version of Bret M.'s original post
(http://home.comcast.net/~bretm/hash/3.html) and have validated
that this is the same paper that is originally cited.
diff --git a/doc/internals/hashing.txt b/doc/internals/hashing.txt
index 281dcf6..1bf6b26 100644
--- a/doc/internals/hashing.txt
+++ b/doc/internals/hashing.txt
@@ -79,5 +79,5 @@
 applicable and may result in less smooth distribution.
 
 References:
-Mixing Functions/Avalanche: http://home.comcast.net/~bretm/hash/3.html
+Mixing Functions/Avalanche: https://papa.bretmulvey.com/post/124027987928/hash-functions
 Hash Functions: http://www.cse.yorku.ca/~oz/hash.html