Use relative link to config.md

Change-Id: I9aad590ad4a3e794e5aaeef22347fe78679e9128

Documentation/markdown.md

diff --git a/Documentation/markdown.md b/Documentation/markdown.md
index 7b89b4d..daaaff6 100644
--- a/Documentation/markdown.md
+++ b/Documentation/markdown.md
@@ -619,7 +619,7 @@
 _(optional)_
 
 Embedded source URLs must also be whitelisted by the Gitiles
-[`markdown.allowiframe`](/Documentation/config.md#IFrames) configuration
+[`markdown.allowiframe`](config.md#IFrames) configuration
 variable.
 
 ## Site layout