[XEB] Theory notebook: fix pauli vs. depol error number (#4097)
[XEB] Theory notebook: fix pauli vs. depol error number (#4097)

In the XEB theory notebook, the `p**(depth)` line wasn't exactly matching up with the XEB decay curve. This adds notes about the distinction between pauli error and depolarizing error rates and performs the necessary fixup for comparison.

There's also a little math notation tidying. 
quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq