entities: rename section with eth and thorn to "Icelandic"

Eth and thorn are letters in the Icelandic alphabet, and the language
is in active use.

TINYCHANGE

lisp/org-entities.el

@@ -226,7 +226,7 @@ packages to be loaded, add these packages to `org-latex-packages-alist'."
      ("beth" "\\beth" t "ℶ" "beth" "beth" "ב")
      ("dalet" "\\daleth" t "ℸ" "dalet" "dalet" "ד")
 
-     "** Dead languages"
+     "** Icelandic"
      ("ETH" "\\DH{}" nil "Ð" "D" "Ð" "Ð")
      ("eth" "\\dh{}" nil "ð" "dh" "ð" "ð")
      ("THORN" "\\TH{}" nil "Þ" "TH" "Þ" "Þ")