Add netcoreapp3.1

Author: HMBSbige

SingleInstance/SingleInstance.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>netstandard2.0</TargetFramework>
+    <TargetFrameworks>netstandard2.0;netcoreapp3.1</TargetFrameworks>
     <LangVersion>latest</LangVersion>
     <Authors>HMBSbige</Authors>
     <Description>Enforces single instance for an application.</Description>