add number of max characters to the failure dialog in upload check

sd-card/html/ota_page.html

@@ -169,7 +169,7 @@ function upload() {
     } else if (filePath.length == 0) {
         alert("File path on server is not set!");
     } else if (filePath.length > 30) {
-        alert("Filename is to long! Use a shorter name.");
+        alert("Filename is to long! Max 30 characters.");
     } else if (filePath.indexOf(' ') >= 0) {
         alert("File path on server cannot have spaces!");
     } else if (filePath[filePath.length-1] == '/') {