Fix csharp version

csharp/src/Google.Protobuf/Properties/AssemblyInfo.cs

@@ -64,4 +64,4 @@ using System.Security;
 
 [assembly: AssemblyVersion("3.0.0.0")]
 [assembly: AssemblyFileVersion("3.0.0.0")]
-[assembly: AssemblyInformationalVersion("3.0.0-alpha4")]
+[assembly: AssemblyInformationalVersion("3.0.0-beta3")]