DOC: update Tim's address in .mailmap

Let's keep the ASCII-only version that's already used in most commits
and presents well everywhere.
diff --git a/.mailmap b/.mailmap
index 30353d7..f72077f 100644
--- a/.mailmap
+++ b/.mailmap
@@ -1 +1 @@
-Tim Düsterhus <tim@bastelstu.be>
+Tim Duesterhus <tim@bastelstu.be>