Update video_demo.py

[GuangjiHuang]

1. wait-time is the wrong name of the variable;

2. the cv.waitKey's parameter delay must be the interger intead of the float datatype.

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

Please describe the motivation of this PR and the goal you want to achieve through this PR.

Modification

Please briefly describe what modification is made in this PR.

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repos?
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

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMCls.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[CLAassistant]

All committers have signed the CLA.

[GuangjiHuang]

.

[ZwwWayne]

MMDetection use '-' to separate words in command arguments, like resume-from, wait-time to ease the usages. So the change in line 22 should be omitted.

[GuangjiHuang]

Oh, absolutely, you are right! But, there is a situation that when I see the code "args.resume_from", I don't know what the argument is and then I want to jump to the parser.add_argument(...) for more information about the args.reume_from, so in the vim normal mode, I will key "mm*ggn" to jump to the destination code line. If the using the _ to separate the words, I can easily jump to line of the code parser.add_argument, but I can't do that if using the - to separate words.

[ZwwWayne]

I see... The original purpose is to ease the typing in the command line, which we think might be more commonly needed. For your case with VIM, you may directly search for 'wait' rather than 'wait_time'?
For now, we also do not wish break the compatibility of arguments actually

[RangiLyu]

All args in mmdet use - instead of _. It will be converted automatically when parsing args.

Suggested change

	'--wait_time',
	'--wait-time',

[OpenMMLab-Assistant003]

Hi @GuangjiHuang！We are grateful for your efforts in helping improve this open-source project during your personal time.
Welcome to join OpenMMLab Special Interest Group (SIG) private channel on Discord, where you can share your experiences, ideas, and build connections with like-minded peers. To join the SIG channel, simply message moderator— OpenMMLab on Discord or briefly share your open-source contributions in the #introductions channel and we will assist you. Look forward to seeing you there! Join us ：https://discord.gg/UjgXkPWNqA
If you have a WeChat account，welcome to join our community on WeChat. You can add our assistant ：openmmlabwx. Please add "mmsig + Github ID" as a remark when adding friends：）
Thank you again for your contribution❤ @GuangjiHuang