Fix incorrect string format in js build script opencv#26374
I accidentally met this small problem mentioned in opencv#25084 (comment) when play with wasm build. It seems https://github.com/EDVTAZ didn't fix it yet, so I create this tiny pr.
Additionally, I remove a redundant argument in `add_argument` call. `'store_true'` already set the default, see https://docs.python.org/3/library/argparse.html#action.
### Pull Request Readiness Checklist
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- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [x] The PR is proposed to the proper branch
- [x] There is a reference to the original bug report and related work
- [ ] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [ ] The feature is well documented and sample code can be built with the project CMake