Update README.rdoc

Update rubydoc URL
Closes ruby-ldap#384

Author: HarlemSquirrel

README.rdoc

@@ -23,7 +23,7 @@ the most recent LDAP RFCs (4510–4519, plus portions of 4520–4532).
 
 == Synopsis
 
-See {Net::LDAP on rubydoc.info}[https://www.rubydoc.info/gems/net-ldap/Net/LDAP] for documentation and usage samples.
+See {Net::LDAP on rubydoc.info}[https://www.rubydoc.info/github/ruby-ldap/ruby-net-ldap] for documentation and usage samples.
 
 == Requirements