Fix typo in PlexSession data attribute

[JonnyWong16]

Description

Fix typo in PlexSession self._data attribute.

Typo from #1510

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the docstring for new or existing methods
• I have added tests when applicable