doc: use HTTPS URL for the API documentation page

PR-URL: #913
Reviewed-By: Colin Ihrig <[email protected]>

Author: shinnn

README.md

@@ -34,7 +34,7 @@ string which includes their date (in UTC time) and the commit SHA at
 the HEAD of the release.
 
 **API documentation** is available in each release and nightly
-directory under _docs_. <http://iojs.org/api/> points to the the
+directory under _docs_. <https://iojs.org/api/> points to the the
 latest version.
 
 ### Verifying Binaries