add equation for bootweights

[nadiaenh]

Fixes issue #309

[codecov-commenter]

Codecov Report

Merging #312 (dca7b41) into main (735613e) will not change coverage.
The diff coverage is n/a.

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  Lines          242       242           
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  Hits           242       242           

Files Changed	Coverage Δ
src/bootstrap.jl	100.00% <ø> (ø)

[ayushpatnaikgit]

1. The variance equation is correct, but need to add the reference.
2. The weights equation is mysterious. Seems unrelated to bootstrapping.

Here are the correct equations:

pg 385, Section 9.3.3 Bootstrap - Sharon Lohr, Sampling Design and Analysis (2010)