Fix typos in tutorial documentation #29299
This PR fixes spelling typos across several tutorial documentation files (dnn, calib3d, objdetect, app, js_tutorials), including `export=dowload` -> `export=download` in the DNN text-spotting tutorial's Google Drive URLs.
Documentation-only change; no functional code is affected.
Pull Request Readiness Checklist:
[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 code under GPL or another license that is incompatible with OpenCV.
[x] The PR is proposed to the proper branch (4.x).
[x] Documentation-only change; no accuracy/performance tests or opencv_extra patch are applicable.