docs: enhance supported_chars

Author: feng19

lib/captcha.ex

@@ -17,7 +17,7 @@ defmodule Captcha do
   Create by options:
 
       iex> Captcha.create(
-      ...>   set_chars: "1234567890abcdefgABCDEFG",
+      ...>   set_chars: "123456789abcdefgABCDEFG",
       ...>   add_chars: 6,
       ...>   set_color: %{r: 0, g: 116, b: 204},
       ...>   view: %{w: 220, h: 120},
@@ -45,7 +45,7 @@ defmodule Captcha do
   example:
 
       iex> Captcha.create(
-      ...>   set_chars: "1234567890abcdefgABCDEFG",
+      ...>   set_chars: "123456789abcdefgABCDEFG",
       ...>   add_chars: 6,
       ...>   set_color: %{r: 0, g: 116, b: 204},
       ...>   view: %{w: 220, h: 120},
@@ -93,6 +93,10 @@ defmodule Captcha do
   defdelegate create_by_name(captcha_name, difficulty \\ :easy, options \\ nil), to: Native
 
   @doc """
+  Supported Chars
+
+  Not support chars: 0, O, o, L, i
+
   example:
 
       iex> Captcha.supported_chars()

test/captcha_test.exs

@@ -4,7 +4,7 @@ defmodule CaptchaTest do
   alias Captcha.Native.Geometry
   alias Captcha.Native.Filter.{Cow, Dots, Grid, Noise, Wave}
 
-  @chars "1234567890abcdefgABCDEFG"
+  @chars "123456789abcdefgABCDEFG"
 
   test "gen funs" do
     assert {_chars, _png} = Native.easy()