Fix pip install error (#12198)

Fix the error of pip install introduced in PR #12197

Author: yejianquan

dockers/docker-sonic-mgmt/Dockerfile.j2

@@ -255,7 +255,7 @@ RUN python3 -m pip install setuptools-rust \
                             scapy==2.4.5 \
                             celery[redis]==4.4.7 \
                             msrest==0.6.21 \
-                            python-dateutil \
+                            python-dateutil
 
 # Deactivating a virtualenv
 ENV PATH="$BACKUP_OF_PATH"