Fix compiler warning

[mathieucarbou]

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