correct spec intro regarding defined image formats (#52)

The introductory section of "Image format" under "Specification" wrongly
states that there is only one defined image format. This contradicts the
subsequent sections which clearly define both Type 1 and Type 2 (added
in 49a8a33) as distinct formats.

Update the introductory sentence to accurately reflect the existence of
two defined image formats.

Signed-off-by: Ivin Joel Abraham <[email protected]>

Author: ivinjabraham

draft.md

@@ -100,7 +100,7 @@ A file transfer algorithm which implements efficient download of only the conten
 
 ### Image format
 
-The image format determines how an [AppImage] is represented on disk. Currently there is only one defined image format, however, there could be additional image formats in the future.
+The image format determines how an [AppImage] is represented on disk. Currently there are two defined image formats, however, there could be additional image formats in the future.
 
 #### Type 0 image format