[Doc] fixed format of file extension in Readme.md

README.md

@@ -20,7 +20,7 @@ To read the documentation, start by installing the
 [Sphinx](http://sphinx-doc.org) documentation generator tool (just run 
 `easy_install -U Sphinx` from the command line and you're good to go). Once you
  have that, you can build the Swift documentation by going into `docs` and 
-typing `make`.  This compiles the `.rst` files in the `docs` directory into 
+typing `make`.  This compiles the `rst` files in the `docs` directory into 
 HTML in the `docs/_build/html` directory.
 
 Once built, the best place to start is with the Swift white paper, which gives a