fix checkpoint_completion_target

[sebastianwebber]

currently this parameter isn't being computed and on pg 14 the default value changed to 0.9.

[DiegoDAF]

The standard formula is (checkpoint_timeout - 2min) / checkpoint_timeout rounded to one decimal.
I know, the default for checkpoint_timeout is 600s so: 600 - 120 / 600 = 0.8