fix: corrected include directory name for constants/float32/half-pi

[gunjjoshi]

Description

What is the purpose of this pull request?

This pull request:

• renames the include directory name for constants/float32/half-pi

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None.

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@stdlib-js/reviewers

[gunjjoshi]

Sorry for the mistake, the folder had named as float64 instead of float32. I'd be more cautious from now onwards.
This PR is a pre-requisite for #1950.

cc: @kgryte

[kgryte]

LGTM. Thanks for catching!