chore: add missing backticks to flags.js (#19226)

Author: lumirlumir

lib/shared/flags.js

@@ -10,7 +10,7 @@
  */
 const activeFlags = new Map([
     ["test_only", "Used only for testing."],
-    ["unstable_config_lookup_from_file", "Look up eslint.config.js from the file being linted."],
+    ["unstable_config_lookup_from_file", "Look up `eslint.config.js` from the file being linted."],
     ["unstable_ts_config", "Enable TypeScript configuration files."]
 ]);