Fix broken link on example's smithy model (#3725)

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
There's a broken link on the Readme of the example:
https://github.com/smithy-lang/smithy-rs/blob/main/examples/README.md?plain=1#L34

## Description
Fix the link to point to the common-test models instead:
https://github.com/smithy-lang/smithy-rs/blob/main/codegen-core/common-test-models/pokemon.smithy



## Testing
Readme update, no testing

## Checklist
None
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Co-authored-by: Zelda Hessler <zhessler@amazon.com>