[5.0.x] Fixed typo in docs/ref/contrib/admin/index.txt.

Backport of 3cb1ba50ccde5b33d6bc5b7cc1ea22c8af3c2aa3 from main

docs/ref/contrib/admin/index.txt

@@ -1268,7 +1268,7 @@ subclass::
     ======  ====================
     Prefix  Lookup
     ======  ====================
-    ^       :lookup:`startswith`
+    ^       :lookup:`istartswith`
     =       :lookup:`iexact`
     @       :lookup:`search`
     None    :lookup:`icontains`