Fix MD001 warnings

The following warnings are generated by using markdownlint analysis:
```
docs/README.md:10 MD001/heading-increment Heading levels should only increment by one level at a time [Expected: h3; Actual: h4]
```
Refer to markdown-lint [1] to fix MD001
[1]: https://github.com/updownpress/markdown-lint/blob/master/rules/001-header-increment.md

Signed-off-by: George Liu <liuxiwei@ieisystem.com>
Change-Id: I55d5132edb76bd2e46ea34d579b6a7b264581059
1 file changed