Remove nuget preview tag.

Author: SvenGroot

nuget/package/Ookii.CommandLine.Cpp.nuspec

@@ -2,7 +2,7 @@
 <package xmlns="http://schemas.microsoft.com/packaging/2011/08/nuspec.xsd">
   <metadata>
     <id>Ookii.CommandLine.Cpp</id>
-    <version>2.0.0-preview</version>
+    <version>2.0.0</version>
     <title>Ookii.CommandLine for C++</title>
     <authors>Sven Groot</authors>
     <owners>Sven Groot</owners>