Fix build-system.requires cross-reference to dependency specifiers (#1644)

[paneendrakumar0]

Summary

In source/specifications/pyproject-toml.rst, the specification for the [build-system] table previously stated:

This key must have a value of a list of strings representing dependencies required to execute the build system. The strings in this list follow the :ref:version specifier specification <version-specifiers>.

In packaging standards, PEP 518 specifies that build-system.requires contains PEP 508 dependencies (package name + version constraint + markers, e.g., "setuptools>=61.0", "wheel"). These are dependency specifiers, not PEP 440 version specifiers (which solely define version comparison operators like >=61.0).

This PR corrects the text and the Sphinx cross-reference to point to the dependency specifier specification (dependency-specifiers.rst), matching PEP 518, the example given directly beneath it (requires = ["setuptools"]), and the rest of the pyproject.toml specification.

Fixes #1644

Verification

• Built documentation with sphinx-build -n -W source build (0 warnings, 0 errors).
• Verified that the rendered HTML link points to dependency-specifiers.html#dependency-specifiers.
• Verified that pre-commit run --all-files passes all formatting and linting checks cleanly.

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📚 Documentation preview 📚: https://python-packaging-user-guide--2156.org.readthedocs.build/en/2156/