Fix typos in the documentation. (quantumlib#6359)

Author: smburdick

cirq/ops/pauli_gates.py

@@ -186,7 +186,7 @@ def basis(self) -> Dict[int, '_ZEigenState']:
 
     This is the `exponent=1` instance of the `cirq.XPowGate`.
 
-    The untary matrix of `cirq.X` is:
+    The unitary matrix of `cirq.X` is:
     $$
     \begin{bmatrix}
         0 & 1 \\