Fix GitHub issue template (make file end in .md instead of .md.txt)

[bergmeister]

PR Summary

The GitHub issue template added in PR 865 does not get applied because the file accidentally ends in .md.txt (because GitHub appends .txt automatically when downloading raw files).
Changing it to .md should make it work since the PR template works (which ends only in .md)

PR Checklist

Note: Tick the boxes below that apply to this pull request by putting an x between the square brackets. Please mark anything not applicable to this PR NA.

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• NA User facing documentation needed
• NA Change is not breaking
• NA Make sure you've added a new test if existing tests do not effectively test the code changed
• This PR is ready to merge and is not work in progress
  • If the PR is work in progress, please add the prefix WIP: to the beginning of the title and remove the prefix when the PR is ready