tests: Fix docker '--memory' flag in GPU test
tests: Fix docker '--memory' flag in GPU test

A missing space was resulting in an error:

```
invalid argument "18ghacc-test-weekly" for "-m, --memory" flag: invalid size: '18ghacc-test-weekly'
```

Change-Id: I63cd3f1cf877442b1b5fffc0a09024ca6ae94436
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/59489
Maintainer: Bobby Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Bobby Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5