Wrong label in locallang_mask.xlf

[anderlk]

Backend icons are also allowed to be svg graphics. But the label only talks about png graphics. So i suggest to change the label in locallang_mask as follows or similar:
`

Folder for 32 x 32 svg or png preview-images (e.g. mykey.png) for contentelements `

[nhovratov]

Thanks 👍