update hpkp spec reference per @reedloden

Author: oreoshake

README.md

@@ -8,7 +8,7 @@ The gem will automatically apply several headers that are related to security.
 - X-Content-Type-Options - [Prevent content type sniffing](http://msdn.microsoft.com/en-us/library/ie/gg622941\(v=vs.85\).aspx)
 - X-Download-Options - [Prevent file downloads opening](http://msdn.microsoft.com/en-us/library/ie/jj542450(v=vs.85).aspx)
 - X-Permitted-Cross-Domain-Policies - [Restrict Adobe Flash Player's access to data](https://www.adobe.com/devnet/adobe-media-server/articles/cross-domain-xml-for-streaming.html)
-- Public Key Pinning - Pin certificate fingerprints in the browser to prevent man-in-the-middle attacks due to compromised Certificate Authorites. [Public Key Pinnning  Specification](https://tools.ietf.org/html/draft-ietf-websec-key-pinning-21)
+- Public Key Pinning - Pin certificate fingerprints in the browser to prevent man-in-the-middle attacks due to compromised Certificate Authorites. [Public Key Pinnning  Specification](https://tools.ietf.org/html/rfc7469)
 
 ## Usage