[ZEPPELIN-6182] fix fail on running script to enable system service

### What is this PR for?

fixes error on running a script to enable Zeppelin as system service

error message:

```
$ sudo ./bin/zeppelin-systemd-service.sh enable
Failed to enable unit: Unit file zeppelin.service does not exist.
To start Zeppelin using systemd, simply type:
# systemctl start zeppelin

To check the service health:
# systemctl status zeppelin
```

### What type of PR is it?
Bug Fix

### Todos

### What is the Jira issue?

* https://issues.apache.org/jira/browse/ZEPPELIN-6182

### How should this be tested?

* `sudo ./bin/zeppelin-system-service.sh enable` should work without error message.
* `systemctl list-units --all "zeppelin.*" | grep zeppelin` should list zeppelin.service.
   output: `  zeppelin.service loaded inactive dead Apache Zeppelin daemon`
* `sudo ./bin/zeppelin-system-service.sh disable` should work without error message. 
   `systemctl list-units --all "zeppelin.*" | grep zeppelin` should not list zeppelin.service.
   There should be no /etc/systemd/system/zeppelin.service file.
 
### Screenshots (if appropriate)

### Questions:
* Does the license files need to update? No
* Is there breaking changes for older versions? No
* Does this needs documentation? No



Closes #4921 from sh1nj1/fix/system-serivce-sh.

Signed-off-by: Philipp Dallig <philipp.dallig@gmail.com>

Author: sh1nj1

bin/zeppelin-systemd-service.sh

@@ -37,11 +37,11 @@ function enable_systemd_service()
     ZEPPELIN_DIR=$(dirname "${MY_PWD}")
 
     # Copy the unit file.
-    cp "${ZEPPELIN_DIR}"/scripts/systemd/zeppelin.systemd "${SYSTEMD_DIR}"
+    cp "${ZEPPELIN_DIR}"/scripts/systemd/zeppelin.service "${SYSTEMD_DIR}"
 
     # Swap the template variable with the right directory path.
     sed -i -e "s#%ZEPPELIN_DIR%#${ZEPPELIN_DIR}#g;" \
-        "${SYSTEMD_DIR}"/zeppelin.systemd
+        "${SYSTEMD_DIR}"/zeppelin.service
 
     # Set up the unit file.
     systemctl daemon-reload
@@ -63,7 +63,7 @@ function disable_systemd_service()
     # Let's mop up.
     systemctl stop zeppelin.service
     systemctl disable zeppelin.service
-    rm "${SYSMTED_DIR}"/zeppelin.systemd
+    rm "${SYSTEMD_DIR}"/zeppelin.service
     systemctl daemon-reload
     systemctl reset-failed
 
@@ -111,3 +111,4 @@ case "${1}" in
   *)
     echo "${USAGE}"
 esac
+