bug_report.md: add missing blank lines

Isolate each section title and each list from other elements, to make
them more visually distinguishable.  This should also make it easier to
find and edit only the parts that are meant to be edited.

See also the following discussion:
#4468

.github/ISSUE_TEMPLATE/bug_report.md

@@ -6,31 +6,39 @@ labels: ''
 assignees: ''
 
 ---
+
 Write clear, concise and in textual form.
 
 **Bug and expected behavior**
+
 - Describe the bug.
 - What did you expect to happen?
 
 **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**
+
 Steps to reproduce the behavior:
+
 1. Run in bash `firejail PROGRAM`
 2. See error `ERROR`
 3. Click on '....'
 4. Scroll down to '....'
 
 **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**
+
 Other context about the problem like related errors to understand the problem.
 
 **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`)
  - [ ] I have performed a short search for similar issues (to avoid opening a duplicate).