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 <description> is not well formed because
it should contain a paragraph.
2 jobs for lr/appstream-fix in 4 minutes and 7 seconds (queued for 20 seconds)
latest
Status Job ID Name Coverage
  Build
passed #172268
librem5
build:native

00:02:19

 
  Test
passed #172269
librem5
test:native

00:01:48