check-before-deployment: Add info about chronyd/chronyc on CentOS 8

On CentOS 8 Chrony is used by default or NTP services.
- https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/8/html/configuring_basic_system_settings/using-chrony-to-configure-ntp#sect-checking_if_chrony_is_synchronized

check-before-deployment.md

@@ -189,12 +189,30 @@ To check whether the NTP service is installed and whether it synchronizes with t
     Active: active (running) since 一 2017-12-18 13:13:19 CST; 3s ago
     ```
 
+    - If it returns `Unit ntpd.service could not be found.` then try the following:
+
+    {{< copyable "shell-regular" >}}
+
+    ```bash
+    sudo systemctl status cronyd.service
+    ```
+
+    ```
+    chronyd.service - NTP client/server
+    Loaded: loaded (/usr/lib/systemd/system/chronyd.service; enabled; vendor preset: enabled)
+    Active: active (running) since Mon 2021-04-05 09:55:29 EDT; 3 days ago
+    ```
+
 2. Run the `ntpstat` command to check whether the NTP service synchronizes with the NTP server.
 
     > **Note:**
     >
     > For the Ubuntu system, you need to install the `ntpstat` package.
 
+    > **Note:**
+    >
+    > On CentOS 8 you need to use `chronyc tracking` instead of `ntpstat` if you are using `chronyd` instead of `ntpd`.
+
     {{< copyable "shell-regular" >}}
 
     ```bash