corrected documentation for registry version

The documentation was saying of installing registry 2.3 but
the tag used is registry:2. This tag gets updated to the recent
versions of registry, and hence, this fix was required.
Closes-Bug: 1598231

Change-Id: I3c37c3564678467402223a48c69a1ae7ccbaeebb

doc/multinode.rst

@@ -18,7 +18,7 @@ Docker layering to optimize push operations. For more information reference
 
 
 The Kolla community recommends using registry 2.3 or later. To deploy registry
-2.3 do the following:
+with version greater than 2.3, do the following:
 
 ::