docs/mux-support.md: README.md: Fix MD041 warnings

The following warnings are generated by using markdownlint analysis:
```
docs/mux-support.md:1 MD041/first-line-heading/first-line-h1 First line in a file should be a top-level heading [Context: "## Mux Support"]
```
Refer to markdown-lint [1] to fix MD041
[1]: https://github.com/updownpress/markdown-lint/blob/master/rules/041-first-line-h1.md

Signed-off-by: George Liu <liuxiwei@ieisystem.com>
Change-Id: Id2dc44fd63b40492a67965c4a51aceeb67b9307d
diff --git a/docs/mux-support.md b/docs/mux-support.md
index 3d0b876..820c99e 100644
--- a/docs/mux-support.md
+++ b/docs/mux-support.md
@@ -1,4 +1,4 @@
-## Mux Support
+# Mux Support
 
 In some hardware designs, multiple UARTS may be available behind a Mux, which
 requires obmc-console to select one at a time.