wiki: prepare_image.sh: Drop fx from format

Workaround for this bug:
ImageMagick/ImageMagick#7491

Change-Id: Id0888457d1d69f6492f7322dcf27e0d73966af5e

Author: SebaUbuntu

scripts/prepare_image.sh

@@ -27,7 +27,7 @@ fi
 # Trim the image
 mogrify -path $IMAGE_DIR -trim +repage $IMAGE_PATH
 
-RESULT=$(identify -format "%[fx:w]x%[fx:h]" $IMAGE_PATH)
+RESULT=$(identify -format "%[w]x%[h]" $IMAGE_PATH)
 WIDTH=$(echo $RESULT | cut -d 'x' -f 1)
 HEIGHT=$(echo $RESULT | cut -d 'x' -f 2)