Fix Julia alias in docs

[oubiwann]

The latest release of the rouge syntax library doesn't have the new support for Julia in it. The work was completed 10 days ago:

• Add Julia syntax highlighting rouge-ruby/rouge#320

and merged 2 days later:

• Create lexer for the Julia programming language rouge-ruby/rouge#331

The next release should have it. When we update, we'll need to change the Julia alias in our docs to point to julia instead of vb (which it's using right now).