fix: Fix broken postinst reported by @wsldankers.

debian/jitsi-meet-prosody.postinst

 
         # Old versions of jitsi-meet-prosody come with the extra plugin path commented out (https://github.com/jitsi/jitsi-meet/commit/e11d4d3101e5228bf956a69a9e8da73d0aee7949)
         # Make sure it is uncommented, as it contains required modules.
-        if grep -q '--plugin_paths = { "/usr/share/jitsi-meet/prosody-plugins/" }' ;then
+        if grep -q -- '--plugin_paths = { "/usr/share/jitsi-meet/prosody-plugins/" }' $PROSODY_HOST_CONFIG ;then
             sed -i 's#--plugin_paths = { "/usr/share/jitsi-meet/prosody-plugins/" }#plugin_paths = { "/usr/share/jitsi-meet/prosody-plugins/" }#g' $PROSODY_HOST_CONFIG
             PROSODY_CONFIG_PRESENT="false"
         fi