Update doc/fluid/beginners_guide/install/install_Docker_en.md

Co-Authored-By: NHao Wang <31058429+haowang101779990@users.noreply.github.com>

doc/fluid/beginners_guide/install/install_Docker_en.md

@@ -87,5 +87,5 @@ After entering Docker, execute the following commands:
 
 * ***GPU version PaddlePaddle***: `pip uninstall paddlepaddle-gpu`
 
-or use `docker rm [Name of container]` to delete Docker container directly.
+or use `docker rm [Name of container]` to delete a Docker container directly.