docs: fix the type of plugin_for (#2128)

Signed-off-by: Robert Minsk <robertminsk@yahoo.com>

Author: cfxegbert

docs/source/package_definition.rst

@@ -753,14 +753,14 @@ the data type, and includes a code snippet.
       name = "maya_utils"
 
 .. pkgdef:attribute:: plugin_for
-   :type: str
+   :type: list[str]
 
    Provided if this package is a plugin of another package. For example, this might be a maya plugin.
    This is useful when using the :ref:`rez-plugins` command. Also, see :pkgdef:attr:`has_plugins`.
 
    .. code-block:: python
 
-      plugin_for = "maya"
+      plugin_for = ["maya"]
 
 .. pkgdef:function:: post_commands() -> None