[FAB-4484] Fix link to user guide

The link in the README was pointing to
the old link for the fabric-ca.  This
changes the link to point to the correct
RTD for fabric-ca

Change-Id: I5b99f0408dacbd19eca3de7d3782091b3a3989a9
Signed-off-by: Gari Singh <gari.r.singh@gmail.com>

README.md

@@ -2,7 +2,7 @@
 
 This is the Developer's Guide for Fabric CA, which is a Certificate Authority for Hyperledger Fabric.
 
-See [User's Guide for Fabric CA](https://hyperledger-fabric.readthedocs.io/en/latest/Setup/ca-setup.html) for information on how to use Fabric CA.
+See [User's Guide for Fabric CA](https://hyperledger-fabric-ca.readthedocs.io) for information on how to use Fabric CA.
 
 ## Prerequisites