Update hero image in the pub dev example

[AcarFurkan]

Description

Issue reported within #91

If we copied the wolt_modal_bottom sheet directly instead of cloning the example from pub.dev, an error would occur because we did not import the assets but only the code. That's why I decided to pull the example from the github repo instead of getting the assets from local. (This bug was solved before, but I solved it again because it did not fit the guidle.)

Error	Fixed

Related Issues

fix #91

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• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• My PR includes tests for all changed/updated/fixed behaviors.
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
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Breaking Change

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• No, this is not a breaking change.

[yunusemrebakir]

Thanks for the contribution @AcarFurkan!