Incorrect Default Session Table Name

[jeremylivingston]

The default table name in the PdoSessionHandler is "sessions" (plural).

The documentation refers to the table in numerous parts as a singular "session" table. It documents the default value as "session" when it should actually be "sessions". The schema creation scripts also use the wrong table name.

These invalid references should be updated in the docs to preserve BC in the code.

https://github.com/symfony/symfony-docs/blob/master/cookbook/configuration/pdo_session_storage.rst