Fix explain help text

Makes sense. Let's put in the multiline version. 



> On Nov 8, 2014, at 10:15 AM, Dain Sundstrom <notifications@github.com> wrote:
> 
> In presto-cli/src/main/java/com/facebook/presto/cli/Help.java:
> 
> > @@ -22,7 +22,7 @@ public static String getHelpText()
> >          return "" +
> >                  "Supported commands:\n" +
> >                  "QUIT\n" +
> > -                "EXPLAIN [FORMAT {TEXT | GRAPHVIZ}] <query>\n" +
> > +                "EXPLAIN [ ( [FORMAT {TEXT | GRAPHVIZ}] [, TYPE { LOGICAL | DISTRIBUTED }] ) ] <query>\n" +
> Anything is fine with me. I keep having to look up the format, so anything is better then the current, incorrect text.
> 
> —
> Reply to this email directly or view it on GitHub.
>