Replace shell completion request string literal with cobra constant

Signed-off-by: Ben Krieger <ben.krieger@intel.com>

cmd/minikube/cmd/kubectl.go

@@ -96,7 +96,7 @@ host. Please be aware that when using --ssh all paths will apply to the remote m
 			os.Exit(1)
 		}
 
-		if len(args) > 1 && args[0] != "--help" && args[0] != "__complete" {
+		if len(args) > 1 && args[0] != "--help" && args[0] != cobra.ShellCompRequestCmd {
 			cluster := []string{"--cluster", cname}
 			args = append(cluster, args...)
 		}