Replace invalid whitespace in PULL_REQUEST_TEMPLATE.md (#810)

Summary:
I kept getting a weird description when opening PRs to the repo, so I located and replaced an invalid whitespace in the PULL_REQUEST_TEMPLATE.md

Pull Request resolved: #810

Test Plan: N/A

Reviewed By: GijsWeterings

Differential Revision: D35844502

Pulled By: robhogan

fbshipit-source-id: 23b562c45513bd3e9344487c9e8067aaf9db6b2a

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,6 +1,6 @@
 <!-- Thanks for submitting a pull request! Please provide enough information so that others can review your pull request. The two fields below are mandatory. -->
 
-## Summary
+## Summary
 
 <!-- Explain the **motivation** for making this change. What existing problem does the pull request solve? -->