Fix the code examples and the grammatic in `declarative_tables.rst` (#12721)

Fix the code examples and the grammatic in `declarative_tables.rst` (#12721)

* Fix the code example in `declarative_tables.rst`

Add an import of a `Optional` class from the `typing` library in the
code example of the "Union types inside the Type Map" chapter.

* Fix the code example in `declarative_tables.rst`

Replace the import of the `deferred` function to `column_property` from
the `sqlalchemy.orm` package in first code example of the "Applying
Load, Persistence and Mapping Options for Imperative Table Columns"
chapter.

* Fix the grammatic in `declarative_tables.rst`

Remove the unnecessary article in the second paragraph of the
"Mapping to an Explicit Set of Primary Key Columns" chapter in
`declarative_tables.rst`.