chore: update doc headings

Update doc headings from UpperCase(<sub-command>) to LowerCase(<base-command> <sub-command>).
E.g. a command like `juju add-controller` would receive a title of `ADD-CONTROLLER` but is now `juju add-controller` to better reflect the CLI usage.
This shouldn't affect Juju documentation because there the title is often excluded since each command is documented on a different page, but for other tools using this package the title is more useful.

documentation.go

@@ -357,7 +357,7 @@ func (c *documentationCommand) linkForCommand(cmd string) string {
 func (c *documentationCommand) formatCommand(ref commandReference, title bool, commandSeq []string) string {
 	var fmtedTitle string
 	if title {
-		fmtedTitle = strings.ToUpper(strings.Join(commandSeq[1:], " "))
+		fmtedTitle = strings.ToLower(strings.Join(commandSeq, " "))
 	}
 
 	var buf bytes.Buffer