Fix equation typo in qaoa.cycle loss Hamiltonian docstring

[anthayes92]

As titled.

[github-actions]

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Please edit .github/CHANGELOG.md with:

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[trbromley]

Thanks @anthayes92! Still worth updating the changelog, the documentation section has similar entries for typos.

[codecov]

Codecov Report

Merging #1396 (4d2fb9a) into master (92c0ae7) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1396   +/-   ##
=======================================
  Coverage   98.19%   98.19%           
=======================================
  Files         157      157           
  Lines       11729    11729           
=======================================
  Hits        11517    11517           
  Misses        212      212           

Impacted Files	Coverage Δ
pennylane/qaoa/cycle.py	100.00% <ø> (ø)

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[anthayes92]

@trbromley my editor has auto-formatted the changelog again. Thought it's worth mentioning. If it looks ok to you I'll merge this in.