bug_report.md: add checked item example

It is not uncommon for people to use other characters to try to mark an
item as checked (which usually screws up the html output), so be sure to
include an example with "[x]".

.github/ISSUE_TEMPLATE/bug_report.md

@@ -44,6 +44,12 @@ _Any other detail that may help to understand/debug the problem_
 
 ### Checklist
 
+<!--
+Note: Items are checked with an "x", like so:
+
+- [x] This is a checked item.
+-->
+
 - [ ] The issues is caused by firejail (i.e. running the program by path (e.g. `/usr/bin/vlc`) "fixes" it).
 - [ ] I can reproduce the issue without custom modifications (e.g. globals.local).
 - [ ] The program has a profile. (If not, request one in `https://github.com/netblue30/firejail/issues/1139`)