backup.sh -p parameter is for password, should use --port

[fiddleatwork]

mongo --help:

connection options: -h, --host= mongodb host to connect to (setname/host1,host2 for replica sets) --port= server port (can also use --host hostname:port)

authentication options: -u, --username= username for authentication -p, --password= password for authentication --authenticationDatabase= database that holds the user's credentials --authenticationMechanism= authentication mechanism to use

it seems -p is used for password. the script should use --port

I'd submit a PR but it seems the is little or no activity in this repo (several other PR's are still open).