build: switch to motivation/modification wording for template (#2972)

#### Motivation
I find it easier to understand what type of comments to put in each
section when I see Motivation/Modification.



#### Modification
Change the pull request template.



#### Checklist
*If not applicable, provide explanation of why.*
- [ ] Tests updated
- [ ] Docs updated
- [ ] Issue linked in Title

.github/pull_request_template.md

@@ -1,9 +1,9 @@
-#### Description
+#### Motivation
 *What does this change aim to achieve?*
 
 
 
-#### Intention
+#### Modification
 *Why is this change being made? What implications or other considerations are there?*