Fix documentation example for ABC Size (#3985)

The given code example has an ABC size of 6.4, but comments list the
calculation as:

Math.sqrt(1*1 + 5*5 + 2*2)

which results in: 5.48.

The correct calculation for the given the example are:

Math.sqrt(1*1 + 6*6 + 2*2) = 6.40

spec/rubocop/cop/metrics/abc_size_spec.rb

@@ -57,7 +57,7 @@
                       'end'])
       expect(cop.messages)
         .to eq(['Assignment Branch Condition size for method_name is too ' \
-                'high. [6.4/0]']) # sqrt(1*1 + 5*5 + 2*2) => 6.4
+                'high. [6.4/0]']) # sqrt(1*1 + 6*6 + 2*2) => 6.4
     end
 
     context 'target_ruby_version >= 2.3', :ruby23 do