Fix docstring typo

Author: hugovk

src/PIL/Image.py

@@ -326,7 +326,7 @@ def getmodebands(mode: str) -> int:
 
 def preinit() -> None:
     """
-    Explicitly loads BMP, GIF, JPEG, PPM and PPM file format drivers.
+    Explicitly loads BMP, GIF, JPEG, PPM and PNG file format drivers.
 
     It is called when opening or saving images.
     """