fix(changelog_cli): use single asterix for bold text in slack-markdown printer

Author: orestesgaolin

changelog_cli/lib/src/printers/slack_markdown_printer.dart

@@ -37,7 +37,7 @@ class SlackMarkdownPrinter extends Printer {
           buffer.write('- ');
           if (entry.conventionalCommit.scopes.isNotEmpty) {
             final scopes = entry.conventionalCommit.scopes.join(', ');
-            buffer.write('**$scopes**: ');
+            buffer.write('*$scopes*: ');
           }
 
           var message = entry.message;