treewide: fix "an" before consonant U sounds

The article "a" goes before consonant sounds and "an" goes before vowel
sounds. This commit changes an to a for UKI, UDP, UTF-8, URL, UUID, U-Label, UI
and USB, since they start with the sound /ˌjuː/.

src/sysupdate/sysupdate-transfer.c

@@ -316,7 +316,7 @@ static int config_parse_resource_path(
         }
 
         /* Note that we don't validate the path as being absolute or normalized. We'll do that in
-         * transfer_read_definition() as we might not know yet whether Path refers to an URL or a file system
+         * transfer_read_definition() as we might not know yet whether Path refers to a URL or a file system
          * path. */
 
         rr->path_auto = false;