Fixed mininet.org url in README.md.

s/http:\/\/openflow.org\/mininet/http:\/\/mininet.org/g

README.md

@@ -103,7 +103,7 @@ See `INSTALL` for installation instructions and details.
 In addition to the API documentation (`make doc`), much useful
 information, including a Mininet walkthrough and an introduction
 to the Python API, is available on the
-[Mininet Web Site](http://openflow.org/mininet).
+[Mininet Web Site](http://mininet.org).
 There is also a wiki which you are encouraged to read and to
 contribute to, particularly the Frequently Asked Questions (FAQ.)