Quick hack to make symbol lookup work for C#.

Summary: This is kind of a quick hack to make symbol lookup work for C#. ctags calls C# 'csharp', while pygments recognises it as 'cs' (or at least, I have to put 'cs' in the Arcanist indexed languages for the clickables to appear, while it's 'csharp' in the symbol database).

Test Plan: Tested this in my live install and it makes symbol lookup work.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7497