Fix link in README

Author: WardBrian

README.md

@@ -1,5 +1,5 @@
 # stan-website
-The Stan project's web pages:  [mc-stan.org](https:/mc-stan.org)
+The Stan project's web pages:  [mc-stan.org](https://mc-stan.org)
   + How to install Stan
   + The Stan ecosystem of tools
   + Learning Stan: tutorials, case-studies, and field guides