Update pull request template to place motivation before description (#2589)

Pull requests read more intuitively when they begin with the motivation
before diving into a detailed description.

Author: Bennett-Lynch

.changes/next-release/documentation-AWSSDKforJavav2-da6813f.json

@@ -0,0 +1,6 @@
+{
+    "category": "AWS SDK for Java v2", 
+    "contributor": "Bennett-Lynch", 
+    "type": "documentation", 
+    "description": "Update pull request template to place motivation before description"
+}

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,12 +1,12 @@
 <!--- Provide a general summary of your changes in the Title above -->
 
-## Description
-<!--- Describe your changes in detail -->
-
 ## Motivation and Context
 <!--- Why is this change required? What problem does it solve? -->
 <!--- If it fixes an open issue, please link to the issue here -->
 
+## Description
+<!--- Describe your changes in detail -->
+
 ## Testing
 <!--- Please describe in detail how you tested your changes -->
 <!--- Include details of your testing environment, and the tests you ran to -->