update fleet doc

doc/fluid/api/fleet.rst

-==================
-paddle.distributed.fleet
-==================
+=================
+distributed.fleet
+=================
 
 ..  toctree::
     :maxdepth: 1
 
-    fleet/Fleet.rst
+    fleet/DatasetFactory.rst
     fleet/DistributedStrategy.rst
+    fleet/Fleet.rst
     fleet/PaddleCloudRoleMaker.rst
-    fleet/DatasetFactory.rst
     fleet/UserDefinedRoleMaker.rst
     fleet/UtilBase.rst

doc/fluid/api/paddle.rst

-=======================
-paddle
-=======================
+==================
+paddle.distributed
+==================
 
 ..  toctree::
     :maxdepth: 1
@@ -23,7 +23,6 @@ paddle
     paddle/cast.rst
     paddle/ceil.rst
     paddle/cholesky.rst
-	paddle/chunk.rst
     paddle/clamp.rst
     paddle/CompiledProgram.rst
     paddle/concat.rst
@@ -42,7 +41,7 @@ paddle
     paddle/default_startup_program.rst
     paddle/diag.rst
     paddle/disable_imperative.rst
-	paddle/dist.rst
+    paddle/distributed.rst
     paddle/distribution.rst
     paddle/div.rst
     paddle/dot.rst
@@ -66,6 +65,7 @@ paddle
     paddle/eye.rst
     paddle/fill_constant.rst
     paddle/flatten.rst
+    paddle/fleet.rst
     paddle/flip.rst
     paddle/floor.rst
     paddle/full.rst
@@ -98,7 +98,6 @@ paddle
     paddle/logical_xor.rst
     paddle/logsumexp.rst
     paddle/manual_seed.rst
-	paddle/masked_select.rst
     paddle/matmul.rst
     paddle/max.rst
     paddle/maximum.rst
@@ -115,7 +114,6 @@ paddle
     paddle/not_equal.rst
     paddle/ones.rst
     paddle/ones_like.rst
-	paddle/numel.rst
     paddle/ParallelExecutor.rst
     paddle/ParamAttr.rst
     paddle/pow.rst