doc: Improve gphase unitary matrix definition in docstring

[rmshaffer]

Issue #, if available:
LaTeX rendering is incorrect for gphase unitary matrix definition.

see: https://amazon-braket-sdk-python.readthedocs.io/en/latest/_apidoc/braket.circuits.gates.html#braket.circuits.gates.GPhase

Description of changes:
Fix LaTeX markup. Also add full matrix definition for consistency with other gates. With these changes, it looks like:

Testing done:
Validated rendered docs from PR output.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (64a2536) to head (a50ec4b).

Additional details and impacted files

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##              main      #944   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          133       133           
  Lines         8909      8909           
  Branches      2008      2008           
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  Hits          8909      8909           

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[speller26]

Is it potentially misleading to depict the global phase as a 2*2 matrix?

[rmshaffer]

Is it potentially misleading to depict the global phase as a 2*2 matrix?

Yes, not only misleading but completely wrong. Thanks for catching this, not sure why I did that. Opened #946 to fix it.