Remove reference of deprecated decorator from the docs

The `tasks` page contained a reference of `@bot.init_shared_memory`,
which was deprecated in botogram 0.2.

Author: Pietro Albini

docs/tasks.rst

@@ -25,7 +25,7 @@ implement this, with a resolution of one second. Just use the
 
 .. code-block:: python
 
-   @bot.init_shared_memory
+   @bot.prepare_memory
    def init(shared):
        shared["subs"] = []