Fixed link to documentation

Author: phw

README.md

@@ -10,7 +10,7 @@ the library can extract the MCN/UPC/EAN and the ISRCs from disc.
 ruby-discid supports all features from libdiscid 0.6, but will also work with
 any earlier version. If a feature is unsupported in the libdiscid version used
 ruby-discid will just return a default value. The version required for a feature
-is documented in the [API documentation](http://www.rubydoc.info/github/phw/ruby-discid/master).
+is documented in the [API documentation](https://www.rubydoc.info/gems/discid/).
 
 ## Requirements
 * Ruby >= 2.1