appdata.xml: fix a validation error
appstream-util version 0.7.16 complains:
$ appstream-util validate-relax --nonet ./data/sm.puri.Calls.appdata.xml
./data/sm.puri.Calls.appdata.xml: FAILED:
• tag-missing : <description> required
Validation of files failed
What it actually means that the is not well formed because it should contain a paragraph.