Fix documentation for PhasedXZGate (#6870)

* Fix documentation for PhasedXZGate

* Update phased_x_z_gate.py

---------

Co-authored-by: Michael Hucka <[email protected]>

Author: dabacon

cirq-core/cirq/ops/phased_x_z_gate.py

@@ -70,9 +70,9 @@ def __init__(
 
     @classmethod
     def from_zyz_angles(cls, z0_rad: float, y_rad: float, z1_rad: float) -> 'cirq.PhasedXZGate':
-        """Create a PhasedXZGate from ZYZ angles.
+        r"""Create a PhasedXZGate from ZYZ angles.
 
-        The returned gate is equivalent to $Rz(z0_rad) Ry(y_rad) Rz(z1_rad)$ (in time order).
+        The returned gate is equivalent to $Rz(z0\_rad) Ry(y\_rad) Rz(z1\_rad)$ (in time order).
         """
         return cls.from_zyz_exponents(z0=z0_rad / np.pi, y=y_rad / np.pi, z1=z1_rad / np.pi)