it's -> its ;) (#992)

README.md

@@ -54,7 +54,7 @@ API documentation can be found in the main [jimp package](./packages/jimp)
 - [cover](./packages/plugin-cover) - Scale the image so the given width and height keeping the aspect ratio.
 - [displace](./packages/plugin-displace) - Displaces the image based on a displacement map
 - [dither](./packages/plugin-dither) - Apply a dither effect to an image.
-- [flip](./packages/plugin-flip) - Flip an image along it's x or y axis.
+- [flip](./packages/plugin-flip) - Flip an image along its x or y axis.
 - [gaussian](./packages/plugin-gaussian) - Hardcore blur.
 - [invert](./packages/plugin-invert) - Invert an images colors
 - [mask](./packages/plugin-mask) - Mask one image with another.