Fix package license warning

Author: devlead

src/LitJson/LitJSON.csproj

@@ -23,7 +23,7 @@ It's quick and lean, without external dependencies.</Description>
     <Copyright>The authors disclaim copyright to this source code.</Copyright>
     <Authors>Leonardo Boshell, Mattias Karlsson and contributors</Authors>
     <Company>Leonardo Boshell, Mattias Karlsson and contributors</Company>
-    <PackageLicenseUrl>https://github.com/LitJSON/litjson/blob/develop/COPYING</PackageLicenseUrl>
+    <PackageLicenseExpression>Unlicense</PackageLicenseExpression>
     <PackageIconUrl>https://litjson.net/assets/img/logo.png</PackageIconUrl>
     <RepositoryUrl>https://github.com/LitJSON/litjson</RepositoryUrl>
     <RepositoryType>git</RepositoryType>