Change MG_MAX_RECV_BUF_SIZE -> MG_MAX_RECV_SIZE

examples/file-upload/web_root/index.html

@@ -23,7 +23,7 @@
         a partial HTTP message has been received (or a chunk-encoded chunk).
         Use <code>mg_http_delete_chunk()</code> to release chunk memory.
         When 0-sized chunk is received, that's the end of the message.
-        Use <code>MG_MAX_RECV_BUF_SIZE</code> build constant to limit
+        Use <code>MG_MAX_RECV_SIZE</code> build constant to limit
         maximum chunk size on a server side.
         <br><br>
         In this example, JavaScript code uses "fetch()" browser API.