docs(content): correct the family default to rhel=dvd, not EL=dvd

tests/content/content_fixtures.yml

@@ -1,6 +1,6 @@
 ---
 # Raw system.content input -> expected normalized system_cfg.content. Covers family
-# source defaults (EL=dvd, else mirror), url pass-through, satellite inputs, and the
+# source defaults (rhel=dvd, else mirror), url pass-through, satellite inputs, and the
 # Arch reflector lock-out (dropped only when a custom mirror url is set).
 content_fixtures:
   - name: arch-mirror-url-drops-reflector