Correct unitary in the docstring of the FSimGate

[bramathon]

In the docstring for the FSimGate, the unitary is reported to be:

    a = cos(theta)
    b = -1j*sin(theta)
    c = exp(+1j*phi)
        [
            [1, 0, 0, 0],
            [0, a, b, 0],
            [0, b, a, 0],
            [0, 0, 0, c],
        ]

However, the actual unitary produced by the gate is slightly different. Specifically, the term c is exp(-1j*phi). The snippet below demonstrates:

import numpy as np
import cirq

theta = np.pi/4
phi = np.pi/6

# Unitary of FSIM gate is:
        
cirq.unitary(cirq.FSimGate(theta, phi))

actual_c = cirq.unitary(cirq.FSimGate(theta, phi))[3,3]
doc_c = np.exp(1j*phi)

print(f"FSimGate element (3,3) is {actual_c:.4f}, Documentation is {doc_c:.4f}")

The snippet reports that the value of c is 0.8660-0.5000j while the documentation indicates it should be 0.8660+0.5000j.

I believe the gate definition is correct and the documentation is wrong for the following reasons:

1. The documentation also states that the FSim is equal to FSimGate(θ, φ) = ISWAP**(-2θ/π) CZPowGate(exponent=-φ/π) which is consistent with c=exp(-i*φ).
2. The PhasedFSimGate docstring has the conditional phase term as exp(-i*φ) and it would make sense for FSim and PhasedFSim to be consistent.

Thus, this MR modifies the docstring to reflect the unitary produced by the gate.

[NoureldinYosri]

Hi @bramathon thanks for catching this.

[codecov]

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Patch coverage has no change and project coverage change: -0.01% ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6288      +/-   ##
==========================================
- Coverage   97.88%   97.88%   -0.01%     
==========================================
  Files        1104     1104              
  Lines       95819    95819              
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- Hits        93794    93793       -1     
- Misses       2025     2026       +1     

Files Changed	Coverage Δ
cirq-core/cirq/ops/fsim_gate.py	100.00% <ø> (ø)

... and 1 file with indirect coverage changes

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