Use fetchall() method as INSERT example in README

fetchone() may not throw an exception correctly.
#95

Author: ebyhr

README.md

@@ -94,9 +94,9 @@ with trino.dbapi.connect(
 ) as conn:
   cur = conn.cursor()
   cur.execute('INSERT INTO sometable VALUES (1, 2, 3)')
-  cur.fetchone()
+  cur.fetchall()
   cur.execute('INSERT INTO sometable VALUES (4, 5, 6)')
-  cur.fetchone()
+  cur.fetchall()
 ```
 
 The transaction is created when the first SQL statement is executed.