docs/presence/README.md: Fix MD040 warnings

The following warnings are generated by using markdownlint analysis:
```
MD040/fenced-code-language Fenced code blocks should have a language specified [Context: "```"]
```
Refer to markdown-lint [1] to fix MD040
[1]: https://github.com/updownpress/markdown-lint/blob/master/rules/040-fenced-code-language.md

Signed-off-by: George Liu <liuxiwei@ieisystem.com>
Change-Id: Icb601fbd199f536600d35322b7aca19371be00a4
diff --git a/docs/presence/README.md b/docs/presence/README.md
index b54fd8d..c5cc185 100644
--- a/docs/presence/README.md
+++ b/docs/presence/README.md
@@ -110,7 +110,7 @@
 presence is detected and the policy used to update its present state within
 inventory.
 
-```
+```json
 [
   {
     "name": "fan0",
@@ -156,7 +156,7 @@
 
 Example:
 
-```
+```json
 [
   {
     "comments": [