Update webRequest.StreamFilter.write() page (mdn#35861)

Use str.replaceAll() instead of str.replace()

files/en-us/mozilla/add-ons/webextensions/api/webrequest/streamfilter/write/index.md

@@ -46,7 +46,7 @@ function listener(details) {
     let str = decoder.decode(event.data, { stream: true });
     // Just change any instance of Example in the HTTP response
     // to WebExtension Example.
-    str = str.replace(/Example/g, "WebExtension Example");
+    str = str.replaceAll("Example", "WebExtension Example");
     filter.write(encoder.encode(str));
     filter.disconnect();
   };