Typo in mapserverless in docker run command

Nice work on setting up the layer, although I had some trouble running the commands from the README.md

```
$ docker run --rm -v $PWD:/home/mapserverless \
	-it mapsesrverless package.sh
```

needs to be

```
$ docker run --rm -v $PWD:/home/mapserverless \
	-it mapserverless package.sh
```