[3.10] Fix docstring typo in sqlite3.Connection.executescript/sqlite3.Cursor.executescript (GH-27147) (GH-27151)

Both `executescript` methods contain the same docstring typo:
_"Executes a multiple SQL statements at once."_ => _"Executes multiple SQL statements at once."_

Co-authored-by: Erlend Egeberg Aasland <erlend.aasland@innova.no>