Refine PULL_REQUEST_TEMPLATE.md (#13304)

* Refine PULL_REQUEST_TEMPLATE.md

* Update .github/PULL_REQUEST_TEMPLATE.md

Co-authored-by: Subhramit Basu <subhramit.bb@live.in>

---------

Co-authored-by: Subhramit Basu <subhramit.bb@live.in>

Author: koppor

.github/PULL_REQUEST_TEMPLATE.md

@@ -19,7 +19,7 @@ Go through the checklist below. It is mandatory, even for a draft pull request.
 
 Keep ALL the items. Replace the dots inside [.] and mark them as follows: 
 [x] done 
-[ ] not done 
+[ ] TODO (yet to be done)
 [/] not applicable
 -->