update coverage from 10x to 20x per description in documentation page

[taylorreiter]

Data description states:

Below is a discussion of a synthetic set of test cases using three randomly generated (fake) genomes of the same size, with two even read data set abundances of 2x each, and a third read data set with 20x.

But then says:

First, we make some synthetic data sets:

    r1.fa with 2x coverage of genome s10
    r2.fa with 10x coverage of genome s10.
    r3.fa with 2x coverage of genome s11.

I think s10 should be 20x coverage, so I changed it to that.

@bluegenes @ctb ready for review & merge

[codecov]

Codecov Report

Merging #1736 (a86c911) into latest (4bcb274) will increase coverage by 7.38%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           latest    #1736      +/-   ##
==========================================
+ Coverage   82.72%   90.10%   +7.38%     
==========================================
  Files         114       87      -27     
  Lines       12200     8398    -3802     
  Branches     1555     1555              
==========================================
- Hits        10092     7567    -2525     
+ Misses       1850      573    -1277     
  Partials      258      258              

Flag	Coverage Δ
python	90.10% <ø> (ø)
rust	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/core/src/index/bigsi.rs
src/core/src/lib.rs
src/core/src/sketch/minhash.rs
src/core/src/index/search.rs
src/core/src/ffi/nodegraph.rs
src/core/tests/test.rs
src/core/src/encodings.rs
src/core/src/cmd.rs
src/core/src/ffi/minhash.rs
src/core/src/ffi/hyperloglog.rs
... and 17 more

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[bluegenes]

This looks right to me

[ctb]

agree - see a86c911.

note that the listed command just below the tests comment string change actually specifies -C 20 so it's clearly 20x coverage being generated! nice catch, @taylorreiter!

let's merge when tests pass!