Docs: Have gulp.lastRun take a function to avoid task registration (#1828)

Author: jakearchibald

README.md

@@ -205,7 +205,7 @@ const paths = {
 }
 
 function images() {
-  return gulp.src(paths.images.src, {since: gulp.lastRun('images')})
+  return gulp.src(paths.images.src, {since: gulp.lastRun(images)})
     .pipe(imagemin({optimizationLevel: 5}))
     .pipe(gulp.dest(paths.images.dest));
 }