docs: update formatUpdated

[Koooooo-7]

Summary

Add the config formatUpdated: '{YYYY}/{MM}/{DD} {HH}:{mm}' in docs to make this plugin more clearly.

What kind of change does this PR introduce?

Docs

For any code change,

• Related documentation has been updated if needed
• Related tests have been updated or tests have been added

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

Related issue, if any:

Tested in the following browsers:

• Chrome
• Firefox
• Safari
• Edge
• IE

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[trusktr]

It doesn't appear to work, if you check on the bottom of any page in the preview, you will see this:

[Koooooo-7]

@trusktr
yea, I also checked the preview. but it works well locally. It seems some other issues here.

[sy-records]

Documents can add formatUpdated, but it is not necessary to add Last modified to index.html.

Because we use GitHub Pages, there are problems, see #1157

[Koooooo-7]

@sy-records cool, I remove the code changes. and I will take a look on how to make it works as well.

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