Fix typo in PR template

[gschaffner]

Changes

Fixed a typo in the PR template.

Checklist

If this is a user-facing code change, like a bugfix or a new feature, please ensure that
the you've fulfilled the following conditions (where applicable):

• You've added tests (in tests/) added which would fail without your patch
• You've updated the documentation (in docs/, in case of behavior changes or new
  features)
• You've added a new changelog entry (in docs/versionhistory.rst).

If this is a trivial change, like a typo fix or a code reformatting, then you can ignore
these instructions.

[agronholm]

Thanks!