Fix 'filename' collision between csharp{,-fr}.html.markdown

Both were set to LearnCSharp.cs, so the live site has been serving the French version for English.

(see set_rawcode: https://github.com/adambard/learnxinyminutes-site/blob/547a620dd8dc78f8518b0072e456ae2d384a0465/config.rb#L103)