docs: use significand mask in math/base/special/rempio2

[gunjjoshi]

Description

What is the purpose of this pull request?

This pull request:

• uses STDLIB_CONSTANT_FLOAT64_HIGH_WORD_SIGNIFICAND_MASK, instead of STDLIB_CONSTANT_FLOAT64_HIGH_WORD_SIGN_MASK, as used in both the javascript implementation and in the FreeBSD reference, in math/base/special/rempio2.
• Reference: https://svnweb.freebsd.org/base/release/12.2.0/lib/msun/src/e_rem_pio2.c?revision=367086&view=markup#l69

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None.

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• Read, understood, and followed the contributing guidelines.

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@stdlib-js/reviewers

[Planeshifter]

Good catch!

[gunjjoshi]

Thanks @Planeshifter, this was troubling other C implementations. Now as this is caught, we can work on others.