modify readMe bug (#2542)

* modify readMe bug

* modify readMe bug v1.0

Author: Kelang-Tian

docker/dev/README.md

@@ -7,10 +7,10 @@ code of SQLFlow.
 
 ## Build the DevBox Image
 
-In this directory, run the following command.
+In the root directory of this project, run the following command.
 
 ```bash
-docker build -t sqlflow:dev .
+docker build -t sqlflow:dev -f docker/dev/Dockerfile .
 ```
 
 ## Build SQLFlow from Source Code