Switch to PackageLicenseExpression

Author: bartelink

src/FSharp.UMX.fsproj

@@ -4,7 +4,7 @@
   <PropertyGroup>
     <Authors>@alfonsogarciacaro @eiriktsarpalis</Authors>
     <Description>F# Units of Measure for primitive non-numeric types. Compatible with Fable.</Description>
-    <PackageLicense>MIT</PackageLicense>
+    <PackageLicenseExpression>MIT</PackageLicenseExpression>
     <PackageProjectUrl>https://github.com/fsprojects/FSharp.UMX</PackageProjectUrl>
     <PackageTags>fsharp unitofmeasure fable</PackageTags>