MAINT: Minor touch-ups to GitHub PULL_REQUEST_TEMPLATE (pandas-dev#17207)

Remove leading space from task-list so that tasks aren't nested.

Author: dhimmel

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,4 +1,4 @@
- - [ ] closes #xxxx
- - [ ] tests added / passed
- - [ ] passes ``git diff upstream/master -u -- "*.py" | flake8 --diff``
- - [ ] whatsnew entry
+- [ ] closes #xxxx
+- [ ] tests added / passed
+- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
+- [ ] whatsnew entry