Bump BenchmarkDotNet from 0.15.5 to 0.15.6 (#924)

---
updated-dependencies:
- dependency-name: BenchmarkDotNet
  dependency-version: 0.15.6
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

tests/NSubstitute.Benchmarks/NSubstitute.Benchmarks.csproj

@@ -6,7 +6,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="BenchmarkDotNet" Version="0.15.5" />
+    <PackageReference Include="BenchmarkDotNet" Version="0.15.6" />
   </ItemGroup>
 
   <ItemGroup>