Fix executemany documenation example

Author: elprans

asyncpg/connection.py

@@ -157,7 +157,7 @@ async def executemany(self, command: str, args, timeout: float=None):
 
             >>> await con.executemany('''
             ...     INSERT INTO mytab (a) VALUES ($1, $2, $3);
-            ... ''')
+            ... ''', [(1, 2, 3), (4, 5, 6)])
 
         :param command: Command to execute.
         :args: An iterable containing sequences of arguments.