Update documentation of malformed URL replacement string

Since Soy version 2016-08-09, malformed URLs are replaced with:

  about:invalid#zSoyz

Change-Id: Iaa1089e602eda4a28f06b2b80ece239eb0dd0251

Documentation/markdown.md

diff --git a/Documentation/markdown.md b/Documentation/markdown.md
index baf728f..7b89b4d 100644
--- a/Documentation/markdown.md
+++ b/Documentation/markdown.md
@@ -341,7 +341,7 @@
 
 Well formed URLs beginning with `https://`, `http://`, and `mailto:`
 are used as written for the link's destination.  Malformed URLs may be
-replaced with `#zSoyz` to prevent browser evaluation of dangerous
+replaced with `about:invalid#zSoyz` to prevent browser evaluation of dangerous
 content.
 
 HTML escaping of URL characters such as `&` is handled internally by