Use csharp instead of c# for code blocks for the package's README.md. (#259)

NuGet doesn't seem to recognize `c#` and doesn't apply syntax highlighting for it, whereas `csharp` seems to work - see https://www.nuget.org/packages/MySqlConnector for an example.

Author: kpreisser

src/README.md

@@ -33,7 +33,7 @@ $ dotnet add package wasmtime
 
 Replace the contents of `Program.cs` with the following code:
 
-```c#
+```csharp
 using System;
 using Wasmtime;