fix generateImages cli argument length read

test/Util/generateImages_browserless.js

@@ -28,7 +28,7 @@ function sleep (ms) {
 // global variables
 //   (without these being global, we'd have to pass many of these values to the generateSampleImage function)
 // eslint-disable-next-line prefer-const
-let [osmdBuildDir, sampleDir, imageDir, imageFormat, pageWidth, pageHeight, filterRegex, mode, debugSleepTimeString] = process.argv.slice(2, 10);
+let [osmdBuildDir, sampleDir, imageDir, imageFormat, pageWidth, pageHeight, filterRegex, mode, debugSleepTimeString] = process.argv.slice(2, 11);
 if (!osmdBuildDir || !sampleDir || !imageDir || (imageFormat !== "png" && imageFormat !== "svg")) {
     console.log("usage: " +
         // eslint-disable-next-line max-len