Fix lingering Markdown for git-send-email.io

The link to git-send-email.io wasn't using AsciiDoc syntax, so it didn't get linkified properly.

Author: noelleleigh

book/05-distributed-git/sections/contributing.asc

@@ -787,7 +787,7 @@ Result: OK
 
 [TIP]
 ====
-For help on configuring your system and email, more tips and tricks, and a sandbox to send a trial patch via email, go to [git-send-email.io](https://git-send-email.io/).
+For help on configuring your system and email, more tips and tricks, and a sandbox to send a trial patch via email, go to https://git-send-email.io/[git-send-email.io].
 ====
 
 ==== Summary