Use `csharp` instead of `c#` for code blocks for the package's README.md

[kpreisser]

NuGet doesn't seem to recognize c# as language for code blocks and doesn't apply syntax highlighting for it:

Using csharp instead seems to work - see https://www.nuget.org/packages/MySqlConnector with https://github.com/mysql-net/MySqlConnector/blob/master/src/MySqlConnector/docs/README.md for an example.

Thanks!