Refine String.slice/3 return type to t (#15553)

The spec declared `grapheme` instead, whose typedoc is
"Multiple code points that may be perceived as a single
character", but it may return multiple graphemes.

`grapheme` is defined as `@type grapheme :: t`, so this is
identical to analysis tools.

Author: nseaSeb

lib/elixir/lib/string.ex

@@ -2410,7 +2410,7 @@ defmodule String do
       ""
 
   """
-  @spec slice(t, integer, non_neg_integer) :: grapheme
+  @spec slice(t, integer, non_neg_integer) :: t
 
   def slice(_, _, 0) do
     ""