chore: Update PULL_REQUEST_TEMPLATE.md
Fix a minor grammatical error in the pull request template
## Description, Motivation and Context
-Please include a summary of the change or what problem does it solve. Please also include relevant motivation and context.
+Please include a summary of the change or what problem it solves. Please also include relevant motivation and context.
<!--- If it relates to an open issue, please link to the issue here.
e.g.
