Remove use of deprecated clusterctl flag in docs

Author: Jan Roehrich

docs/book/src/clusterctl/commands/describe-cluster.md

@@ -26,7 +26,7 @@ bootstrap object linked to a machine, unless their state differs from the machin
 By default, the visualization generated by `clusterctl describe cluster` hides details for the sake
 of simplicity and shortness. However, if required, the user can ask for showing all the detail:
 
-By using the `--disable-grouping` flag, the user can force the visualization to show all the machines
+By using `--grouping=false`, the user can force the visualization to show all the machines
 on separated lines, no matter if they have the same state or not:
 
 ![](../../images/describe-cluster-disable-grouping.png)