Update UnitsNet.Benchmark.csproj

there is no end to it...
dotnet/BenchmarkDotNet#1420

Author: lipchev

UnitsNet.Benchmark/UnitsNet.Benchmark.csproj

@@ -8,7 +8,8 @@
     <AssemblyVersion>4.0.0.0</AssemblyVersion> <!-- Should reflect major part of Version -->
     <AssemblyTitle>UnitsNet.Benchmark</AssemblyTitle>
     <Product>UnitsNet.Benchmark</Product>
-    <LangVersion>latest</LangVersion>
+    <LangVersion>8.0</LangVersion>
+    <!-- <LangVersion>latest</LangVersion> -->
     <RootNamespace>UnitsNet</RootNamespace>
   </PropertyGroup>