docs/configuration.md: Fix MD025 warnings

The following warnings are generated by using markdownlint analysis:
```
docs/configuration.md:25 MD025/single-title/single-h1 Multiple top-level headings in the same document [Context: "# IPMI D-Bus Sensor Filtering"]
```
Refer to markdown-lint [1] to fix MD025
[1]: https://github.com/updownpress/markdown-lint/blob/master/rules/025-single-h1.md

Signed-off-by: George Liu <liuxiwei@ieisystem.com>
Change-Id: I2cda186ec6ec17ae049649761cd2b8bc81c3306e