Update bug_report.md

Author: simontreny

.github/ISSUE_TEMPLATE/bug_report.md

@@ -7,6 +7,8 @@ assignees: ''
 
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+<!-- Please search existing issues to avoid creating duplicates (opened and closed). -->
+
 ### Describe the bug
 <!-- A clear and concise description of what the bug is. -->
 <!-- Attach screenshots if possible. -->