Update PR template

Author: soimkim

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,9 +1,12 @@
 ## Description
-(Please describe what this PR do.)
-
+<!--
+Please describe what this PR do.
+ -->
 
 ## Type of change
+<!--
 Please insert 'x' one of the type of change.
+ -->
 - [ ] Bug fix (non-breaking change which fixes an issue)
 - [ ] New feature (non-breaking change which adds functionality)
 - [ ] Documentation update