docs: replace "dropdpwn" with "dropdown"

[goxberry]

The fully-qualified name for the sphinx_panels.dropdown.DropdownDirective is misspelled in the project documentation. This pull request corrects that misspelling.

Signed-off-by: Geoffrey M Oxberry goxberry@gmail.com

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Codecov Report

Base: 84.48% // Head: 84.48% // No change to project coverage :thumbsup:

Coverage data is based on head (ef33abe) compared to base (8fb6f3d). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #50 +/- ## ======================================= Coverage 84.48% 84.48% ======================================= Files 10 10 Lines 1637 1637 ======================================= Hits 1383 1383 Misses 254 254 ``` | Flag | Coverage Δ | | |---|---|---| | pytests | `84.48% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=executablebooks#carryforward-flags-in-the-pull-request-comment) to find out more. Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=executablebooks). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=executablebooks)

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