fix: Correcting name of variable from table_schema to schema_name (#1114)

Author: ankiaga

google/cloud/spanner_dbapi/_helpers.py

@@ -24,7 +24,7 @@
 SQL_GET_TABLE_COLUMN_SCHEMA = """
 SELECT COLUMN_NAME, IS_NULLABLE, SPANNER_TYPE
 FROM INFORMATION_SCHEMA.COLUMNS
-WHERE TABLE_SCHEMA = @table_schema AND TABLE_NAME = @table_name
+WHERE TABLE_SCHEMA = @schema_name AND TABLE_NAME = @table_name
 """
 
 # This table maps spanner_types to Spanner's data type sizes as per