Fix kubectl exec syntax (#2916)

Author: dveeden

en/enable-tls-between-components.md

@@ -1631,7 +1631,7 @@ In this step, you need to perform the following operations:
     {{< copyable "shell-regular" >}}
 
     ``` shell
-    kubectl exec -it ${cluster_name}-pd-0 -n ${namespace} sh
+    kubectl exec -it ${cluster_name}-pd-0 -n ${namespace} -- sh
     ```
 
     Use `pd-ctl`:
@@ -1650,7 +1650,7 @@ In this step, you need to perform the following operations:
     {{< copyable "shell-regular" >}}
 
     ``` shell
-    kubectl exec -it ${cluster_name}-tikv-0 -n ${namespace} sh
+    kubectl exec -it ${cluster_name}-tikv-0 -n ${namespace} -- sh
     ```
 
     Use `tikv-ctl`:

en/enable-tls-for-dm.md

@@ -548,7 +548,7 @@ Get into the DM-master Pod:
 {{< copyable "shell-regular" >}}
 
 ``` shell
-kubectl exec -it ${cluster_name}-dm-master-0 -n ${namespace} sh
+kubectl exec -it ${cluster_name}-dm-master-0 -n ${namespace} -- sh
 ```
 
 Use `dmctl`:

zh/enable-tls-between-components.md

@@ -1617,7 +1617,7 @@ aliases: ['/docs-cn/tidb-in-kubernetes/dev/enable-tls-between-components/']
     {{< copyable "shell-regular" >}}
 
     ``` shell
-    kubectl exec -it ${cluster_name}-pd-0 -n ${namespace} sh
+    kubectl exec -it ${cluster_name}-pd-0 -n ${namespace} -- sh
     ```
 
     使用 `pd-ctl`：
@@ -1636,7 +1636,7 @@ aliases: ['/docs-cn/tidb-in-kubernetes/dev/enable-tls-between-components/']
     {{< copyable "shell-regular" >}}
 
     ``` shell
-    kubectl exec -it ${cluster_name}-tikv-0 -n ${namespace} sh
+    kubectl exec -it ${cluster_name}-tikv-0 -n ${namespace} -- sh
     ```
 
     使用 `tikv-ctl`：

zh/enable-tls-for-dm.md

@@ -521,7 +521,7 @@ spec:
 {{< copyable "shell-regular" >}}
 
 ``` shell
-kubectl exec -it ${cluster_name}-dm-master-0 -n ${namespace} sh
+kubectl exec -it ${cluster_name}-dm-master-0 -n ${namespace} -- sh
 ```
 
 使用 `dmctl`：