Update progress in README

Fixes #6.

Author: bradfeehan

README.md

@@ -19,7 +19,7 @@ PHP client for [Desk.com](http://desk.com) v2 API, based on
     * [Delete](https://github.com/bradfeehan/desk-php/issues/13)
     * [Search](https://github.com/bradfeehan/desk-php/issues/15)
  * Sub-item operations (ListCaseNotes, etc)
-    * [Show](https://github.com/bradfeehan/desk-php/issues/6)
+    * ~~[Show](https://github.com/bradfeehan/desk-php/issues/6)~~
     * ~~[List](https://github.com/bradfeehan/desk-php/issues/4)~~
     * [Create](https://github.com/bradfeehan/desk-php/issues/9)
     * [Update](https://github.com/bradfeehan/desk-php/issues/12)