Fix pull request template formatting (#2288)

The pull request box is not formatted in the same way as other Markdown around
the GitHub site, so looking at the preview after the last changes to the
template gave the wrong impression.

Author: ttuegel

.github/PULL_REQUEST_TEMPLATE.md

@@ -4,25 +4,9 @@ Fixes #nnnn
 
 ###### Review checklist
 
-The author performs the actions on the checklist.
-The reviewer evaluates the work and checks the boxes as they are completed.
+The author performs the actions on the checklist. The reviewer evaluates the work and checks the boxes as they are completed.
 
--   [ ] **Summary**
-    Write a summary of the changes.
-    Explain what you did to fix the issue, and why you did it.
-    Present the changes in a logical order.
-    Instead of writing a summary in the pull request,
-    you may push a clean Git history.
--   [ ] **Documentation**
-    Write documentation for new functions.
-    Update documentation for functions that changed,
-    or complete documentation where it is missing.
--   [ ] **Tests**
-    Write unit tests for every change.
-    Write the unit tests that were missing before the changes.
-    Include any examples from the reported issue as integration tests.
--   [ ] **Clean up**
-    The changes are already clean.
-    Clean up anything near the changes that you noticed while working.
-    This does not mean only spatially near the changes, but logically near:
-    any code that interacts with the changes!
+-   [ ] **Summary.** Write a summary of the changes. Explain what you did to fix the issue, and why you did it. Present the changes in a logical order. Instead of writing a summary in the pull request, you may push a clean Git history.
+-   [ ] **Documentation.** Write documentation for new functions. Update documentation for functions that changed, or complete documentation where it is missing.
+-   [ ] **Tests.** Write unit tests for every change. Write the unit tests that were missing before the changes. Include any examples from the reported issue as integration tests.
+-   [ ] **Clean up.** The changes are already clean. Clean up anything near the changes that you noticed while working. This does not mean only spatially near the changes, but logically near: any code that interacts with the changes!