Fix MD034 warnings

The following warnings are generated by using markdownlint analysis:
```
phosphor-power-supply/README.md:4:1 MD034/no-bare-urls Bare URL used [Context: "https://github.com/openbmc/doc..."]
```
Refer to markdown-lint [1] to fix MD034
[1]: https://github.com/updownpress/markdown-lint/blob/master/rules/034-no-bare-urls.md

Signed-off-by: George Liu <liuxiwei@ieisystem.com>
Change-Id: I3b7451b50ae2ed3ba9d9374f869025f947e01ce2
diff --git a/phosphor-power-supply/README.md b/phosphor-power-supply/README.md
index 3cb29f3..d26217d 100644
--- a/phosphor-power-supply/README.md
+++ b/phosphor-power-supply/README.md
@@ -1,7 +1,7 @@
 # OpenBMC power supply monitoring application
 
 Design document:
-https://github.com/openbmc/docs/blob/master/designs/psu-monitoring.md
+<https://github.com/openbmc/docs/blob/master/designs/psu-monitoring.md>
 
 ## Compile Options