Fix doc of fleet api (#29282)

* fix doc, test=document_fix

python/paddle/distributed/fleet/base/fleet_base.py

@@ -93,7 +93,7 @@ class Fleet(object):
             paddle.enable_static()
             import paddle.distributed.fleet as fleet
             strategy = fleet.DistributedStrategy()
-            fleet.init(strategy)
+            fleet.init(strategy=strategy)
 
             optimizer = paddle.optimizer.SGD(learning_rate=0.001)
             optimizer = fleet.distributed_optimizer(optimizer)
@@ -176,7 +176,7 @@ class Fleet(object):
 
                 import paddle.distributed.fleet as fleet
                 strategy = fleet.DistributedStrategy()
-                fleet.init(strategy)
+                fleet.init(strategy=strategy)
 
         """