Use headers for bug-report template (elastic#990)

.github/ISSUE_TEMPLATE/Bug_report.md

@@ -4,23 +4,31 @@ about: Create a report to help us improve
 
 ---
 
-**Describe the bug**
+## Describe the bug
+<!--
 A clear and concise description of what the bug is.
+-->
 
-**To Reproduce**
+## Steps to reproduce
+<!--
 Steps to reproduce the behavior:
 1. Use this config '...'
 2. Then call '....'
 3. Then do '....'
 4. See error
+-->
 
-**Expected behavior**
+## Expected behavior
+<!--
 A clear and concise description of what you expected to happen.
+-->
 
-**Debug logs**
+## Debug logs
+<!--
 Attach your debug logs.
 See the [documentation](https://www.elastic.co/guide/en/apm/agent/java/current/trouble-shooting.html#trouble-shooting-logging)
 about how to enable debug logging.
+-->
 
 <details>
 <summary>Click to expand</summary>