Remove erroneous LaTeX alignment character

[1ucian0]

@mberna found a MathJax syntax problem in https://qiskit.org/documentation/stubs/qiskit.quantum_info.Pauli.html

The problem is that that there is an amp alignment in a single equation.

I dont think the fix needs reno nor changelog entry.

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[coveralls]

Pull Request Test Coverage Report for Build 2380478424

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.005%) to 84.376%

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Totals
Change from base Build 2371064921:	0.005%
Covered Lines:	54539
Relevant Lines:	64638

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💛 - Coveralls