Linux: fix appdata
fix appdata validation on Flathub:
• style-invalid : <caption> cannot end in '.' [Create and use gamut masks to give your images a coherent feel.]
• style-invalid : <caption> is too long [If you're new to digital painting, or want to know more about Krita's possibilities, there's an extensive, up-to-date manual.];longest allowed is 100 chars
• style-invalid : <caption> cannot end in '.' [If you're new to digital painting, or want to know more about Krita's possibilities, there's an extensive, up-to-date manual.]
• style-invalid : <caption> cannot end in '.' [Into animation? Krita provides everything you need for traditional, hand-drawn animation.]
• style-invalid : <caption> cannot end in '.' [Krita is a full-featured digital painting studio.]
• style-invalid : <caption> cannot end in '.' [The startup window now also gives you the latest news about Krita.]
• style-invalid : <caption> cannot end in '.' [There are over ten immensely powerful brush engines.]