[Fix] Update README.md to fix a typo

[lan-qing]

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Motivation

There is a typo in projects/BEVFusion/README.md:

bash tools/dist_train.py projects/BEVFusion/configs/bevfusion_lidar_voxel0075_second_secfpn_8xb4-cyclic-20e_nus-3d.py 8,

where dist_train.py seems should be dist_train.sh

Modification

dist_train.py -> dist_train.sh

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects.
4. The documentation has been modified accordingly, like docstring or example tutorials.

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