docs: fix undefined variable in schema evolution transaction example

[anxkhn]

Rationale for this change

The "schema evolution" section of the Python API docs shows how to evolve a
schema inside a transaction:

with table.transaction() as transaction:
    with transaction.update_schema() as update_schema:
        update.add_column("some_other_field", IntegerType(), "doc")
    # ... Update properties etc

The inner context manager is bound as update_schema, but the body calls
update.add_column(...). The name update is not defined in that scope; it is
left over from the preceding standalone example just above it
(with table.update_schema() as update:). Copy-pasting the transaction snippet
verbatim raises NameError: name 'update' is not defined.

This changes the call to use the bound name update_schema, so the documented
example runs as written. It also matches the already-correct
create_table_transaction example earlier in the same page, which uses
update_schema.add_column(...).

Are these changes tested?

No automated test: this is a one-line documentation fix with no code change. I
verified it manually against the public API: Transaction.update_schema()
returns an UpdateSchema context manager whose add_column(path, field_type, doc=None, ...) signature matches the positional call in the example, so the
corrected snippet executes without error. pre-commit (markdownlint, codespell,
trailing-whitespace/end-of-file) passes on the changed file.

Are there any user-facing changes?

No. Documentation only; there are no changes to library behavior or public API.