Fix compiler warning

```
src/WebHandlers.cpp: In member function 'virtual void AsyncStaticWebHandler::handleRequest(AsyncWebServerRequest*)':
src/WebHandlers.cpp:237:38: warning: format '%x' expects argument of type 'unsigned int', but argument 4 has type 'uint32_t' {aka 'long unsigned int'} [-Wformat=]
  237 |     snprintf(etag, sizeof(etag), "%08x", etagValue);
      |                                   ~~~^   ~~~~~~~~~
      |                                      |   |
      |                                      |   uint32_t {aka long unsigned int}
      |                                      unsigned int
      |                                   %08lx
```

Author: mathieucarbou

src/WebHandlers.cpp

@@ -234,7 +234,7 @@ void AsyncStaticWebHandler::handleRequest(AsyncWebServerRequest *request) {
       size_t fileSize = request->_tempFile.size();
       etagValue = static_cast<uint32_t>(fileSize);
     }
-    snprintf(etag, sizeof(etag), "%08x", etagValue);
+    snprintf(etag, sizeof(etag), "%08lx", etagValue);
   } else {
     etag[0] = '\0';
   }