Update README.md

Author: nilportugues

README.md

@@ -7,7 +7,7 @@
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-PSR7 Response implementation for the [Problem Details for HTTP APIs](http://tools.ietf.org/html/draft-nottingham-http-problem-07)  specification draft.  
+PSR7 Response implementation for the [Problem Details for HTTP APIs (RFC7807)](https://tools.ietf.org/html/rfc7807)  specification.  
 
 ## Usage