bug_report.md: turn section titles into headers

This properly separates the document into sections and makes the titles
stand out more.  It is also more consistent with what is generated by
GitHub issue forms (YAML).

See also the following discussion:
#4468

.github/ISSUE_TEMPLATE/bug_report.md

@@ -9,17 +9,17 @@ assignees: ''
 
 Write clear, concise and in textual form.
 
-**Bug and expected behavior**
+### Bug and expected behavior
 
 - Describe the bug.
 - What did you expect to happen?
 
-**No profile and disabling firejail**
+### No profile and disabling firejail
 
 - What changed calling `firejail --noprofile /path/to/program` in a terminal?
 - What changed calling the program by path (e.g. `/usr/bin/vlc`)?
 
-**Reproduce**
+### Reproduce
 
 Steps to reproduce the behavior:
 
@@ -28,16 +28,16 @@ Steps to reproduce the behavior:
 3. Click on '....'
 4. Scroll down to '....'
 
-**Environment**
+### Environment
 
  - Linux distribution and version (ie output of `lsb_release -a`, `screenfetch` or `cat /etc/os-release`)
  - Firejail version (output of `firejail --version`) exclusive or used git commit (`git rev-parse HEAD`)
 
-**Additional context**
+### Additional context
 
 Other context about the problem like related errors to understand the problem.
 
-**Checklist**
+### Checklist
 
  - [ ] The profile (and redirect profile if exists) hasn't already been fixed [upstream](https://github.com/netblue30/firejail/tree/master/etc).
  - [ ] The program has a profile. (If not, request one in `https://github.com/netblue30/firejail/issues/1139`)