doc: fix GPI2 gate matrix representation

[math411]

Issue #, if available:

Description of changes: The matrix representation was missing the scalar multiplier. Added and checked the docs.

Gate definitions: https://ionq.com/docs/getting-started-with-native-gates#introducing-the-native-gates

Testing done:

tox -e docs

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (d6209ee) to head (20bdcb5).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #919 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 132 132 Lines 8823 8823 Branches 1986 1986 ========================================= Hits 8823 8823 ```

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