Update USAGE.md (#467)

Updating Usage file to fix the following issue:
https://github.com/danielgatis/rembg/issues/452

When replacing the background color, we require RGBA value, and all of that values should range from 0-255.

USAGE.md

@@ -48,7 +48,7 @@ output = remove(input, post_process_mask=True)
 ### Replacing the background color
 You can use the `bgcolor` argument to replace the background color.
 ```python
-output = remove(input, bgcolor=(255, 255, 255))
+output = remove(input, bgcolor=(255, 255, 255, 255))
 ```
 
 ### Using input points