[databases] Fix the beat environments list command

The command example in the docs was wrong.

doc/databases.rst

@@ -407,7 +407,7 @@ Note that there are no "latest" version because of the reproducibility goal of B
 
 In order to list all the BEAT related Docker images on your machine, use the following command:
 
-    $ beat environments list docker
+    $ beat environments list --type docker
 
 To have a more curated list of images, the following command shows only the database related BEAT images: