docs: clarify overview runner for multiple files

[fuguiKz]

Fixes #1306.

The docs/articles/overview.md sample uses BenchmarkRunner.Run(typeof(Program).Assembly), but the text below it referenced BenchmarkRunner.Run<Md5VsSha256>(), which is confusing.

• Align the text with the sample code (Run(typeof(Program).Assembly)).
• Clarify that benchmark classes can live in separate files (they just need to be public and in the same assembly).

Docs-only change; no code changes.

[fuguiKz]

CI note: the previous run (21112065930) failed only on test-macos (x64).

Failure details:

• Job: https://github.com/dotnet/BenchmarkDotNet/actions/runs/21112065930/job/60712132025
• Failing test: DisassemblyDiagnoserTests.CanDisassembleAllMethodCalls(jit: RyuJit, platform: X64, toolchain: .NET 8.0)
• Exception: InvalidOperationException: Sequence contains no elements (DisassemblyDiagnoser.Results was empty; output also said “No benchmarks were disassembled”).
• The benchmark process exited with code 139 (createdump was produced), which likely led to the missing disassembly output.

Other jobs (Windows/Linux/macOS arm64) were green, and the latest run-tests on master is passing, so this looks flaky/environmental.

I pushed an empty commit to retrigger CI (run 21113188459 is queued). If test-macos (x64) keeps failing, could a maintainer please re-run/advise? Thanks!

[timcassell]

Thanks @fuguiKz