Change spelling disc -> disk (#15569)

Disc is typically used for optical media, not hard disks (so unlikely
to encounter discs in mdraid related circumstances).
mdadm and friends use the disk spelling, the host script also shows
shows this `"disc_count": $(maybe_get "${mdadmSysDev}/md/raid_disks"),`

Fixing the variable names as well would be nice, but that change would
be a lot bigger and probably break things as it would need host script
adjustment synchronising and all that, but I think we can at least
change the cosmetic spelling.

https://en.wikipedia.org/wiki/Spelling_of_disc

includes/html/pages/device/apps/mdadm.inc.php

@@ -30,8 +30,8 @@ print_optionbar_end();
 $graphs = [
     'mdadm_level'          => 'RAID level',
     'mdadm_size'           => 'RAID Size',
-    'mdadm_disc_count'     => 'RAID Disc count',
-    'mdadm_hotspare_count' => 'RAID Hotspare Disc count',
+    'mdadm_disc_count'     => 'RAID Disk count',
+    'mdadm_hotspare_count' => 'RAID Hotspare Disk count',
     'mdadm_degraded'       => 'RAID degraded',
     'mdadm_sync_speed'     => 'RAID Sync speed',
     'mdadm_sync_completed' => 'RAID Sync completed',