Docs: change Deep Zoom file extension for libvips v8.4.2+

Author: lovell

docs/api-output.md

@@ -161,7 +161,7 @@ sharp('input.tiff')
   .tile({
     size: 512
   })
-  .toFile('output.dzi', function(err, info) {
+  .toFile('output.dz', function(err, info) {
     // output.dzi is the Deep Zoom XML definition
     // output_files contains 512x512 tiles grouped by zoom level
   });

lib/output.js

@@ -237,7 +237,7 @@ const toFormat = function toFormat (format, options) {
  *   .tile({
  *     size: 512
  *   })
- *   .toFile('output.dzi', function(err, info) {
+ *   .toFile('output.dz', function(err, info) {
  *     // output.dzi is the Deep Zoom XML definition
  *     // output_files contains 512x512 tiles grouped by zoom level
  *   });