Docstring only: Fix math parsing error for Pauli docstring

[grossardt]

Summary

:math:`k`th in docstring (or generally math directly followed by a letter) results in parsing error, see qiskit.quantum_info.Pauli under section 'Array Representation'. Assuming this is a Sphinx bug, but as a simple fix replaced by :math:`k`-th. (:class:qiskit.quantum_info.Pauli seems the only place where this issue occurs.)

(Since this is a simple typo fix in the docstring only, I directly opened the PR without filing a bug report first and without adding release notes. Please let me know if this is improper procedure.)

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[jakelishman]

Thanks for the fix.

It's not a Sphinx bug so much as a documented part of rST. Roles (the `...` bit) need to be followed by a non-word character. Using - like here is ok, as would be using an explicit escape (":math:`k`\\ th" or r":math:`k`\ th" as appropriate).

[coveralls]

Pull Request Test Coverage Report for Build 6866194577

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 5 unchanged lines in 1 file lost coverage.
• Overall coverage remained the same at 85.928%

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Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/lex.rs	5	91.92%

Totals
Change from base Build 6865596785:	0.0%
Covered Lines:	65938
Relevant Lines:	76736

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💛 - Coveralls