Typo: JIFF -> JFIF.

Doc/lib/libimghdr.tex

@@ -30,7 +30,7 @@ return value from \function{what()}:
   \lineii{'tiff'}{TIFF Files}
   \lineii{'rast'}{Sun Raster Files}
   \lineii{'xbm'}{X Bitmap Files}
-  \lineii{'jpeg'}{JPEG data in JIFF format}
+  \lineii{'jpeg'}{JPEG data in JFIF format}
   \lineii{'bmp'}{BMP files}
   \lineii{'png'}{Portable Network Graphics}
 \end{tableii}