Removed trailing spaces that broke commands in GETTING_STARTED.md

[lynnpepin]

Tiny change: Removes trailing spaces from commands in GETTING_STARTED.md

The backslash \ is meant to escape the newline when writing multi-line commands. But because there was a space character after the backslash, the backslash escaped the space and not the newline, and the command would not work properly.

Example:

python -m pcdet.datasets.nuscenes.nuscenes_dataset --func create_nuscenes_infos \ 
    --cfg_file tools/cfgs/dataset_configs/nuscenes_dataset.yaml \
    --version v1.0-trainval