Add variation selector to ❌ in best_practices heading

The heading was missing the FE0F variation selector that forces
emoji presentation. Other pages had it which is why ❌ rendered
in color there but not on this page.

Author: escamoteur

docs/documentation/listen_it/best_practices.md

@@ -52,7 +52,7 @@ print(lazy.value);  // 21 ✅ (NOW updates on next change)
 **Recommendation**: Don't mix. Use all-eager (default, simple) or all-lazy (memory optimization). Mixing can cause hard-to-debug stale values.
 :::
 
-### ❌ WRONG: Chains in Build Methods
+### ❌️ WRONG: Chains in Build Methods
 
 Never create chains inline in build methods: