Update awslogs.md (#5816)

the [ and ] need to be escapped to avoid the folling docker error: 
docker: Error response from daemon: failed to initialize logging driver: awslogs could not parse multiline pattern key "[\\d{4}-(?:0[1-9]|1[0-2])-(?:0[1-9]|[1,2][0-9]|3[0,1])T(?:[0,1][0-9]|2[0-3]):[0-5][0-9]:[0-5][0-9]+00:00]": error parsing regexp: invalid character class range: `}-(`.

engine/admin/logging/awslogs.md

@@ -131,7 +131,7 @@ that expression:
 $ docker run --log-driver=awslogs \
              --log-opt awslogs-region=us-east-1 \
              --log-opt awslogs-group=myLogGroup \
-             --log-opt awslogs-datetime-format='[%b %d, %Y %H:%M:%S]' \
+             --log-opt awslogs-datetime-format='\[%b %d, %Y %H:%M:%S\]' \
              ...
 ```