docs/mux-support.md: Fix MD014 warnings

The following warnings are generated by using markdownlint analysis:
```
MD014/commands-show-output Dollar signs used before commands without showing output [Context: "$ obmc-console-server --config..."]
```
Refer to markdown-lint [1] to fix MD014
[1]: https://github.com/updownpress/markdown-lint/blob/master/rules/014-commands-show-output.md

Signed-off-by: George Liu <liuxiwei@ieisystem.com>
Change-Id: I9daa0403355e45bf39410ccf92693bdccf1830c4
diff --git a/docs/mux-support.md b/docs/mux-support.md
index 0ddf898..30a4850 100644
--- a/docs/mux-support.md
+++ b/docs/mux-support.md
@@ -56,7 +56,7 @@
 and [Example Diagram](#example-diagram)
 
 ```sh
-$ obmc-console-server --config server.conf /dev/ttyS0
+obmc-console-server --config server.conf /dev/ttyS0
 ```
 
 ## Mux Control