update ImageSharp code example to current API

[Sioma112233]

The API of ImageSharp changed a bit and the code example that uses ImageSharp is now outdated.
This fixes this document.

Related also to this issue in a different repo

[frederikja163]

Was the appropriate change to this also made to the source of the tutorails?

[frederikja163]

Also had merge conflicts, can you double check those @Sioma112233

[Sioma112233]

Was the appropriate change to this also made to the source of the tutorails?

Not sure I understand, Isn't this repo the source of the tutorials? (the tutorials in the site opentk.net)

About the merge conflicts, the name of the file was changed so that's the issue.
I'm gonna open a new PR because I already deleted my forked repo so the original branch doesn't exist

[frederikja163]

If you can resolve the merge conflicts i will be happy to merge this. A new website build is up ahead.

[NogginBops]

@frederikja163 you might have missed that you already merged this in #47 .
This is an old PR that isn't relevant anymore. Closing.

[NogginBops]

Ah wait, I spoke too soon. I didn't see that this change was to a different file from #47 .
It seems like I'm not able to re-open this PR, though I think it might be easier to change in a new PR.

Keeping the code of the website and the tutorial repo in sync does seem to be a slight issue.