docs: fix Docker web image command

[enzowilliam]

Summary

Fixes the custom Docker web image example in the Docker deployment guide.

Why

The example currently includes bundle exec twice in the web container CMD. That command is inaccurate and can confuse users who copy the Dockerfile snippet to build custom Chatwoot images.

What this change does

• Removes the duplicated bundle exec from the web Dockerfile example
• Keeps the worker Dockerfile example unchanged

Validation

• Cross-checked against Chatwoot's Docker Compose command for the Rails service
• Ran git diff --check