doc: fix performanceEntry.flags style format

PR-URL: #37274
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/perf_hooks.md

@@ -257,7 +257,7 @@ The type of the performance entry. It may be one of:
 * `'http2'` (Node.js only)
 * `'http'` (Node.js only)
 
-### performanceEntry.flags
+### `performanceEntry.flags`
 <!-- YAML
 added:
  - v13.9.0