Add a sentence to explain how to skip EPG computation in RB manual

docs/manuals/verification/randomized_benchmarking.rst

@@ -208,6 +208,8 @@ The EPGs of two-qubit RB are analyzed with the corrected EPC if available.
 
 Note that ``EPC_corrected`` value is smaller than one of raw ``EPC``, which indicates
 contribution of depolarization from single-qubit error channels.
+If you don't need ``EPG`` value, you can skip its computation by
+``exp_2q.analysis.set_options(gate_error_ratio=False)``.
 
 
 Displaying the RB circuits