Change markdown extension to .md

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

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

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