Fix kubectl_install doc centos default not install bash-completion (#8888)

Signed-off-by: Yuanbin.Chen <cybing4@gmail.com>

content/en/docs/tasks/tools/install-kubectl.md

@@ -217,6 +217,11 @@ The completion script itself is generated by kubectl, so you typically just need
 Common examples are provided here. For more details, consult `kubectl completion -h`.
 
 ### On Linux, using bash
+On CentOS Linux, you may need to install the bash-completion package which is not installed by default.
+
+```shell
+yum install bash-completion -y
+```
 
 To add kubectl autocompletion to your current shell, run `source <(kubectl completion bash)`.