Rename extension back to md

The Maven Doxia plugin (which generates the site) is hardcoded to use
the md extension. This fixes releng/builder sites not building the
Jenkins page.

Change-Id: Ibaf173768a37b751cc3903bc0f89bac18278a3d5
Signed-off-by: Thanh Ha <thanh.ha@linuxfoundation.org>

diff --git a/src/site/markdown/jenkins.markdown b/src/site/markdown/jenkins.md
similarity index 100%
rename from src/site/markdown/jenkins.markdown
rename to src/site/markdown/jenkins.md