Use PackageLicenseExpression (#20)

Author: worldbeater

Source/OxyPlot.Avalonia/OxyPlot.Avalonia.csproj

@@ -4,7 +4,7 @@
     <PackageId>OxyPlot.Avalonia</PackageId>
     <GeneratePackageOnBuild>True</GeneratePackageOnBuild>
     <Description>OxyPlot is a plotting library for .NET. This is a support library for OxyPlot to work with AvaloniaUI.</Description>
-    <PackageLicenseUrl>https://raw.githubusercontent.com/oxyplot/oxyplot-avalonia/master/LICENSE</PackageLicenseUrl>
+    <PackageLicenseExpression>MIT</PackageLicenseExpression>
     <Copyright>OxyPlot contributors</Copyright>
     <PackageProjectUrl>http://oxyplot.org/</PackageProjectUrl>
     <PackageIconUrl>https://raw.githubusercontent.com/oxyplot/oxyplot/develop/Icons/OxyPlot_128.png</PackageIconUrl>