README.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: "$ ./phosphor-led-manager --con..."]
```
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: If8a9428dfc69b788fe780651ac8ab275a188ee9b
diff --git a/README.md b/README.md
index ada629e..9319840 100644
--- a/README.md
+++ b/README.md
@@ -44,7 +44,7 @@
 Then start the program with
 
 ```text
-$ ./phosphor-led-manager --config example.json
+~# ./phosphor-led-manager --config example.json
 ```
 
 ## Dbus interface