Minor pull request template cleanup

Author: atouchet

PULL_REQUEST_TEMPLATE.md

@@ -2,14 +2,14 @@
 
 
 ---
-<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
+<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
 - [ ] `./mach build -d` does not report any errors
 - [ ] `./mach test-tidy` does not report any errors
-- [ ] These changes fix #__ (github issue number if applicable).
+- [ ] These changes fix #___ (GitHub issue number if applicable)
 
 <!-- Either: -->
 - [ ] There are tests for these changes OR
-- [ ] These changes do not require tests because _____
+- [ ] These changes do not require tests because ___
 
 <!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->