README.md typo (#290)

PDF should be PNG?

README.md

@@ -71,7 +71,7 @@ For future update plan, please query [here](https://github.com/continue-revoluti
 1. You should see the output GIF on the output gallery. You can access GIF output at `stable-diffusion-webui/outputs/{txt2img or img2img}-images/AnimateDiff/{yy-mm-dd}`. You can also access image frames at `stable-diffusion-webui/outputs/{txt2img or img2img}-images/{yy-mm-dd}`. You may choose to save frames for each generation into separate directories in `Settings/AnimateDiff`.
 
 ### API
-It is quite similar to the way you use ControlNet. API will return a video in base64 format. In `format`, `PDF` means to save frames to your file system without returning all the frames. If you want your API to return all frames, please add `Frame` to `format` list. For most up-to-date parameters, please read [here](https://github.com/continue-revolution/sd-webui-animatediff/blob/master/scripts/animatediff_ui.py#L26).
+It is quite similar to the way you use ControlNet. API will return a video in base64 format. In `format`, `PNG` means to save frames to your file system without returning all the frames. If you want your API to return all frames, please add `Frame` to `format` list. For most up-to-date parameters, please read [here](https://github.com/continue-revolution/sd-webui-animatediff/blob/master/scripts/animatediff_ui.py#L26).
 ```
 'alwayson_scripts': {
   'AnimateDiff': {