significance level can only be shown in 2-digits

[ghost]

have set group.sample_size(512).significance_level(0.001);, result like this:

    Finished bench [optimized] target(s) in 5.23s

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

Gnuplot not found, using plotters backend
example/A                  time:   [200.41 us 200.89 us 201.38 us]                    
                        change: [+1.9058% +2.1252% +2.3820%] (p = 0.00 < 0.00)
                        Performance has regressed.
example/B                   time:   [209.62 us 209.76 us 209.92 us]                      
                        change: [+0.5592% +0.6910% +0.8199%] (p = 0.00 < 0.00)
                        Change within noise threshold.
example/C                   time:   [210.75 us 211.23 us 211.77 us]                        
                        change: [+4.1680% +4.4006% +4.6917%] (p = 0.00 < 0.00)
                        Performance has regressed.
example/D                   time:   [1.5939 ms 1.6003 ms 1.6079 ms]                      
                        change: [+0.2976% +0.7865% +1.2899%] (p = 0.00 < 0.00)
                        Change within noise threshold.

all (p = 0.00 < 0.00)