issue/PR templates: use h2 instead of h3

`h2` is the semantically correct heading here, as `h1` is for 
document titles and `h2` is for the top level headings within
a document. `h3` should be used for subheadings of `h2` headings.

.github/ISSUE_TEMPLATE/bug_report.md

@@ -4,24 +4,24 @@ about: Report broken or incorrect behaviour
 labels: bug
 ---
 
-### Summary
+## Summary
 
 <!-- A summary of your bug report -->
 
-### Reproduction Steps
+## Reproduction Steps
 
 <!-- What you did to make it happen. Ideally there should be a short code snippet in this section to help reproduce the bug. -->
 
-### Expected Results
+## Expected Results
 
 <!-- What you expected to happen -->
 
-### Actual Results
+## Actual Results
 
 <!-- What actually happened. If there is a traceback, please show the entire thing. -->
 
 
-### Checklist
+## Checklist
 
 <!-- Put an x inside [ ] to check it, like so: [x] -->
 
@@ -29,7 +29,7 @@ labels: bug
 - [ ] I have shown the entire traceback, if possible.
 - [ ] I have removed my token from display, if visible.
 
-### System Information
+## System Information
 
 <!-- Run `python -m discord -v` and paste this information below. -->
 <!-- This command is available in v1.1.0 or higher. If you are unable to run this command, paste basic info (ie. Python version, library version, and your operating system -->