fix: increase margin for sampling distribution test

[katiebayes]

This should do the trick for the bounds that is occasionally not met in tests.
For sample_rate of 10, we've been using a margin of 0.05, which tests for samples to be within 250.00 of the 5000 expected. It's occasionally a few outside of that, leading to very occasional build failure.
Bumping to 0.06 will increase the bounds enough that we're still checking sufficiently without running into this break. (5000 * 0.06 == 300)

[martin308]

I wish we didn't have to do it, but this is better than flaky tests!