Revert ".mdwn -> .markdown"

This reverts commit 3aa9fe332e393f24700a6a14eb256a6d2ac6b93b.

diff --git a/index.markdown b/index.mdwn
similarity index 100%
rename from index.markdown
rename to index.mdwn

diff --git a/index/discussion.markdown b/index/discussion.mdwn
similarity index 100%
rename from index/discussion.markdown
rename to index/discussion.mdwn