Fix read_source.md (#6673)

Optimizer is not a part of fluid. The `paddle::platform`
was missing.

doc/howto/read_source.md

@@ -6,10 +6,10 @@ Core: https://github.com/PaddlePaddle/Paddle/tree/develop/paddle/framework
 
 Operator: https://github.com/PaddlePaddle/Paddle/tree/develop/paddle/operators
 
-Optimizer: https://github.com/PaddlePaddle/Paddle/tree/develop/paddle/optimizer
-
 Memory: https://github.com/PaddlePaddle/Paddle/tree/develop/paddle/memory
 
+Platform: https://github.com/PaddlePaddle/Paddle/tree/develop/paddle/platform
+
 # Compile Time
 
 The following **defines** the NN. The definition goes into this [protocol buffer](https://github.com/PaddlePaddle/Paddle/blob/develop/paddle/framework/framework.proto).