Add space to checkbox markdown in PR template, so they actually render as checkboxes if unedited

[tbarsballe]

From this:

To this:

Related Issue(s):

Closes #

Proposed Changes:

N/A

Diff of User Interface

N/A

PR Checklist:

• This PR is as small and focused as possible
• If this PR includes proposed changes for inclusion in the changelog, the title of this PR summarizes those changes and is ready for inclusion in the Changelog.
• I have updated docstrings for function changes and docs in the 'docs' folder for user interface / behavior changes
• This PR does not break any examples or I have updated them