Fix sign errors in qml.DoubleExcitation documentation.

[glassnotes]

Context: There is an error in the signs of the basis state transforms in the DoubleExcitation documentation; it does not match the actual transformation that is being performed.

Description of the Change: Fixes the signs in relevant places in the docs.

The existing example given is:

dev = qml.device('default.qubit', wires=4)

@qml.qnode(dev)
def circuit(phi):
    qml.PauliX(wires=0)
    qml.PauliX(wires=1)
    qml.DoubleExcitation(phi, wires=[0, 1, 2, 3])
    return qml.state()

>>> circuit(0.1)
tensor([ 0.        +0.j,  0.        +0.j,  0.        +0.j,
        -0.04997917+0.j,  0.        +0.j,  0.        +0.j,
         0.        +0.j,  0.        +0.j,  0.        +0.j,
         0.        +0.j,  0.        +0.j,  0.        +0.j,
         0.99875026+0.j,  0.        +0.j,  0.        +0.j,
         0.        +0.j], requires_grad=True)

If you run this, it produces the state -sin(phi/2)|0011> + cos(phi/2) |1100>, whereas the original documentation displayed both signs as a +.

Benefits: ^ the above 🎉

Possible Drawbacks: None

Related GitHub Issues: None

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[ixfoduap]

Thanks for catching this!

[codecov]

Codecov Report

Merging #2072 (9b28196) into master (8999d50) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2072   +/-   ##
=======================================
  Coverage   99.18%   99.18%           
=======================================
  Files         226      226           
  Lines       17369    17369           
=======================================
  Hits        17228    17228           
  Misses        141      141           

Impacted Files	Coverage Δ
pennylane/ops/qubit/qchem_ops.py	100.00% <ø> (ø)

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