DOC: fix import of tm in Panel dsintro.rst section

[jorisvandenbossche]

xref #23802

[datapythonista]

With the time since Panel was deprecated, not sure if we could remove that section too.

But the fix look good.

[codecov]

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[jorisvandenbossche]

With the time since Panel was deprecated, not sure if we could remove that section too.

It's the section about the deprecation, so I think it is still useful :)