remove the space from the importer artifact name

Previously after the `release-clients` target I was getting,
```
$ ls target/*-mac.zip
target/OMERO.editor-4.4.8-DEV-ice33-mac.zip
target/OMERO.importer -4.4.8-DEV-ice33-mac.zip
target/OMERO.insight-4.4.8-DEV-ice33-mac.zip
```