Fix issue template (nginx#729)

Problem: Enhancement issue template was not showing up in GitHub
Solution: Fix the doc header so it shows up

Author: miledxz

.github/ISSUE_TEMPLATE/enhancement.md

@@ -1,6 +1,7 @@
 ---
-name: Enhancement Request about: Suggest an idea for this project title: ''
-labels: 'proposal' assignees: ''
+name: Enhancement Request 
+about: Suggest an enhancement for this project 
+labels: 'enhancement'
 ---
 
 <!--