Update cortex-with-cassandra.md (#2087)

It seems the latest tag is no longer published. This specifies where version
tags can be found, and uses the current latest in the given command for running
the cortex docker image

Signed-off-by: Blagoj Atanasovski <blagoj@timescale.com>

Author: atanasovskib

docs/guides/cortex-with-cassandra.md

@@ -109,10 +109,13 @@ storage:
     keyspace: KEYSPACE   # configure desired keyspace here.
 ```
 
+The latest tag is not published for the Cortex docker image. Visit quay.io/repository/cortexproject/cortex 
+to find the latest stable version tag and use it in the command bellow. At the time of writing it was v0.6.1. 
+
 Run Cortex using the latest stable version:
 
 ```
-docker run -d --name=cortex -v $(pwd)/single-process-config.yaml:/etc/single-process-config.yaml -p 9009:9009  quay.io/cortexproject/cortex -config.file=/etc/single-process-config.yaml
+docker run -d --name=cortex -v $(pwd)/single-process-config.yaml:/etc/single-process-config.yaml -p 9009:9009  quay.io/cortexproject/cortex:v0.6.1 -config.file=/etc/single-process-config.yaml
 ```
 In case you prefer to run the master version, please follow this [documentation](https://github.com/cortexproject/cortex/blob/master/docs/getting_started.md) on how to build Cortex from source.