Update BenchmarkDotNet to 0.15.8

Author: jstedfast

Benchmarks/Benchmarks.csproj

@@ -7,7 +7,7 @@
 
   <ItemGroup>
     <PackageReference Include="AngleSharp" Version="1.4.0" />
-    <PackageReference Include="BenchmarkDotNet" Version="0.14.0" />
+    <PackageReference Include="BenchmarkDotNet" Version="0.15.8" />
     <PackageReference Include="HtmlPerformanceKit" Version="1.0.0" />
   </ItemGroup>