Set the edit_uri explicitly

I assumed it was derived from the `docs_dir`, it seems it's set
by default to `edit/master/docs/`.

More details - https://www.mkdocs.org/user-guide/configuration/#edit_uri

Author: bbatsov

mkdocs.yml

@@ -1,5 +1,6 @@
 site_name: "CIDER: The Clojure Interactive Development Environment that Rocks"
 repo_url: https://github.com/clojure-emacs/cider
+edit_uri: edit/master/doc/
 site_favicon: favicon.ico
 copyright: "Copyright © 2016-2018 Bozhidar Batsov, Artur Malabarba and CIDER contributors"
 docs_dir: doc