Fix formatting of pull_request_template.md

[srajtiwari]

Current formatting of the pull request template does not auto-populate a link to the issue and does not correctly display checkboxes in the checklist. The following format works well.

# Description
Fixes #nn

Add description here. 

## Dependencies affected:
- 


## Checklist before merging
- [] Properly labeled PR 
- [] Licensing statement added to new files 
- [] Downstream dependencies have been addressed
- [] Corresponding changes to the documentation have been made
- [] Issue is linked under the development section

[micholas]

Thanks for creating this issue :)