refactor(Placeholder.tsx): corrigindo lógica do componente

react/components/Placeholder/Placeholder.tsx

@@ -1,16 +1,29 @@
 const Placeholder = () => {
   if (typeof document !== "undefined") {
+    const observer = new MutationObserver(mutations => {
+      mutations.forEach(_mutation => {
         const postalCodeInput = document.querySelector(
           ".vtex-address-form-4-x-input"
         );
         const postalCodeValue = document.querySelector(".postalCode");
+
         if (postalCodeInput) {
           postalCodeInput.classList.add("postalCode");
+          observer.disconnect();
         }
+
         if (postalCodeValue) {
           postalCodeValue.setAttribute("placeholder", "Digite seu CEP");
         }
+      });
+    });
+
+    observer.observe(document, {
+      childList: true,
+      subtree: true
+    });
   }
+
   return null;
 };