fix example link in readme

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 92.03%. Comparing base (1a9f883) to head (17a898b).

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #290 +/- ## ======================================= Coverage 92.03% 92.03% ======================================= Files 49 49 Lines 9129 9129 ======================================= Hits 8402 8402 Misses 727 727 ``` | [Flag](https://app.codecov.io/gh/rainyl/opencv_dart/pull/290/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=rainy+liu) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/rainyl/opencv_dart/pull/290/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=rainy+liu) | `92.03% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=rainy+liu#carryforward-flags-in-the-pull-request-comment) to find out more.

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