Change Kanuri autonym to lowercase

In Kanuri, the name of the language is specifically
written in lowercase, according to
"Dictionary of the Kanuri Language"
Edited by: Norbert Cyffer and John Hutchinson
De Gruyter 1990.

data/language-data.json

@@ -2141,7 +2141,7 @@
             [
                 "AF"
             ],
-            "Kanuri"
+            "kanuri"
         ],
         "krc": [
             "Cyrl",