Valid markdown syntax for checkboxes

[supernova-at]

Updates PULL_REQUEST_TEMPLATE for proper markdown syntax

[PWAStudioBot]

	Fails
🚫	Missing "Description" section. Please add it back, with detail.
🚫	Missing "Verification Steps" section. Please add it back, with detail.
🚫	No linked issue found. Please link a relevant open issue by adding the text "closes #<issue_number>" or "closes JIRA-<issue_number>" in your PR.
🚫	A version label is required. A maintainer must add one.

	Messages
📖	Access a deployed version of this PR here. Make sure to wait for the "pwa-pull-request-deploy" job to complete.
📖	DangerCI Failures related to missing labels/description/linked issues/etc will persist until the next push or next nightly build run (assuming they are fixed).

If your PR is missing information, check against the original template here. At a minimum you must have the section headers from the template and provide some information in each section.

Generated by 🚫 dangerJS against b3f595b

[devops-pwa-codebuild]

Performance Test Results

The following fails have been reported by WebpageTest. These numbers indicates a possible performance issue with the PR which requires further manual testing to validate.

https://pr-2434.pwa-venia.com : LH Performance Expected 0.85 Actual 0.52, LH Best Practices Expected 1 Actual 0.92
https://pr-2434.pwa-venia.com/venia-tops.html : LH Performance Expected 0.75 Actual 0.37, LH Best Practices Expected 1 Actual 0.92
https://pr-2434.pwa-venia.com/valeria-two-layer-tank.html : LH Performance Expected 0.8 Actual 0.51, LH Accessibility Expected 0.9 Actual 0.89, LH Best Practices Expected 1 Actual 0.92