dont use netcore3.0 yet

Author: Jimmy Byrd

benchmarks/FSharp.Control.Websockets.Benchmarks.fsproj

@@ -2,7 +2,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFrameworks>netcoreapp2.2;netcoreapp3.0</TargetFrameworks>
+    <TargetFrameworks>netcoreapp2.2</TargetFrameworks>
   </PropertyGroup>
   <PropertyGroup Condition="'$(Configuration)'=='Release'">
     <Optimize>true</Optimize>