Fix PackageIcon

Fix according to https://github.com/NuGet/Home/wiki/Packaging-Icon-within-the-nupkg

Author: praeclarum

Directory.Build.props

@@ -3,7 +3,7 @@
         <VersionPrefix>1.8</VersionPrefix>
         <VersionSuffix>beta</VersionSuffix>
 
-        <PackageIconUrl>Logo-low.png</PackageIconUrl>
+        <PackageIcon>Logo-low.png</PackageIcon>
         <PackageProjectUrl>https://github.com/praeclarum/sqlite-net</PackageProjectUrl>
         <RepositoryUrl>https://github.com/praeclarum/sqlite-net.git</RepositoryUrl>
         <PackageTags>sqlite-net;sqlite;database;orm</PackageTags>