fix(autoware_pointcloud_divider): fix the image in README.md

[anhnv3991]

Description

• SInce merge_pcd option is not used anymore, fix the image which describes the naming rules of divided PCD files in README.md

Related links

None

Tests performed

• Old image
  
• Fixed image
  

Notes for reviewers

Just open the preview of autoware_pointcloud_divider's README.md.

Interface changes

None

Effects on system behavior

None

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