doc/contribute.md: remove html markup

Change-Id: Id63ed4217805bc0f4be2d8e4aca09064802abfbb
diff --git a/README.md b/README.md
index 88ca937..32a67ec 100644
--- a/README.md
+++ b/README.md
@@ -81,7 +81,7 @@
 
 Our canonical Git repository is located at https://cue.googlesource.com.
 
-To contribute, please read the [Contribution Guide](./doc/contribute.html).
+To contribute, please read the [Contribution Guide](./doc/contribute.md).
 
 ##### Note that we do not accept pull requests and that we use the issue tracker for bug reports and proposals only.