chore: make fixes field in PR template match auto-close regex (#3520)

Previously, if filling out this template, someone pasted a PR link after
"Fixes #", the issue wouldn't automatically close, probably because the
extra "#" confused the auto-close regex.

Signed-off-by: Will Murphy <willmurphyscode@users.noreply.github.com>

.github/pull_request_template.md

@@ -5,7 +5,7 @@ or link to an issue where this is explained.
 
 <!-- If this completes an issue, please include: -->
 
-- Fixes #
+- Fixes
 
 ## Type of change