Fix code example GroupBy Step to use first letter as per comment

When working through the examples the GroupBy step comment indicates it should be using the first letter of the name, but the ouptut shows it is using the second letter of the name.

The latest version of these docs on github also is using Java as the syntax, yet the codeblock still references Groovy. Since this is Gremlin command line I've changed that back to Groovy syntax.