Set target framework to .NET Standard 2.0 (#88)

Following guidelines at https://learn.microsoft.com/en-us/dotnet/standard/library-guidance/cross-platform-targeting

Author: Mark Whitaker

RegexToolbox/RegexToolbox.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
     <PropertyGroup>
-        <TargetFramework>netstandard1.3</TargetFramework>
+        <TargetFramework>netstandard2.0</TargetFramework>
         <PackageId>RegexToolbox</PackageId>
         <Title>RegexToolbox</Title>
         <Authors>Mark Whitaker</Authors>