Increase kSigmas in sampler_test.cc to reduce flakiness.
Increase kSigmas in sampler_test.cc to reduce flakiness.

Also update comment. I'm not sure why the comment says that the chance of passing when using kSigmas==5 is close to 1-10^-6 because I ran the test 300K times and got 2 failures for kSigmas==5 (which is pretty close to the 3 failures that we would expect if it's actually 1-10^-5).

PiperOrigin-RevId: 449339752
Change-Id: I4b8534803c9c9a845135ebdb6c758ad0306603bc

google/tcmalloc,google/tcmalloc,google/tcmalloc