Improve documentation for Connection.execute

Author: 1st1

asyncpg/connection.py

@@ -124,8 +124,9 @@ async def execute(self, query: str, *args, timeout: float=None) -> str:
 
             >>> await con.execute('''
             ...     CREATE TABLE mytab (a int);
+            ...     INSERT INTO mytab (a) VALUES (100), (200), (300);
             ... ''')
-            CREATE TABLE
+            INSERT 0 3
 
             >>> await con.execute('''
             ...     INSERT INTO mytab (a) VALUES ($1), ($2)