Fixes appstream warnings

There's warnings that only <em> and <code> are permitted in the appdata.xml file xml:

https://appstream.debian.org/sid/main/issues/lollypop.html

This should fix this by replacing <b> with <em>, but this of course changes bold to italic, which might not be wanted. If you want a different fix, please let me know.

See

https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-description

for the specification.