chore(github): update the PR template about code blocks (#85)

.github/PULL_REQUEST_TEMPLATE.md

@@ -21,12 +21,12 @@
 <!--- Follow the <https://keepachangelog.com/en/1.0.0/> format. -->
 <!--- Use one of the Added, Changed, Deprecated, Removed, Fixed, and Security headers accordingly. -->
 <!-- For example:
-```
+````
 ### Added
 
 - Add a middleware for checking the content length
   - Before, the upload size was checked after full upload which was clearly wrong.
-```
+````
 -->
 
 ## Types of Changes