[Fix] fix 'mkdir' error in prepare_voc07_cls.sh

[soonera]

Thanks for your contribution and we appreciate it a lot. The following instructions would make your pull request more healthy and more easily get feedback. If you do not understand some items, don't worry, just make the pull request and seek help from maintainers.

Motivation

When the ' data/ ' directory already exists, ' mkdir data/ ' will report an error.

Modification

Use the -p option. The command should be: ' mkdir -p data/ '.

BC-breaking (Optional)

Does the modification introduce changes that break the backward compatibility of the downstream repositories?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here and update the documentation.

Checklist

Before PR:

• Pre-commit or other linting tools are used to fix the potential lint issues.
• Bug fixes are fully covered by unit tests, the case that causes the bug should be added in the unit tests.
• The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, like docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with those projects, like MMDet or MMSeg.
• CLA has been signed and all committers have signed the CLA in this PR.

[codecov]

Codecov Report

Merging #261 (4cfa044) into dev_v0.8.0 (eec5f23) will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           dev_v0.8.0     #261   +/-   ##
===========================================
  Coverage       69.54%   69.54%           
===========================================
  Files             106      106           
  Lines            3776     3776           
  Branches          607      607           
===========================================
  Hits             2626     2626           
  Misses           1043     1043           
  Partials          107      107           

Flag	Coverage Δ
unittests	69.54% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update eec5f23...4cfa044. Read the comment docs.