website: 'lupio-linux-boot.pmg' -> 'lupio-linux-boot.png'

This typo in the file namewas causing the Linux boot image to not
appear correctly in the LupIO blog post:
https://www.gem5.org/project/2022/02/07/lupio.html

Change-Id: Iff0733f6c1be6ef53dfe3157d2e95738c48a63e2
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/56769
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: Bobby Bruce <bbruce@ucdavis.edu>
diff --git a/assets/img/lupio-linux-boot.pmg b/assets/img/lupio-linux-boot.png
similarity index 100%
rename from assets/img/lupio-linux-boot.pmg
rename to assets/img/lupio-linux-boot.png
Binary files differ