Added Lektor bullet point to PR template

Author: yash

.github/pull_request_template.md

@@ -2,6 +2,7 @@
 
 - [ ] My pull request has a descriptive title (not a vague title like `Update index.md`)
 - [ ] My pull request targets the `master` branch of creativecommons.github.io-source.
+- [ ] I tried building the site using Lektor locally and verified that there are no build errors.
 
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