[MRG] Fix signature filter command in docs.

[mr-eyes]

Adding a missing subcommand filter to the command line docs.

Ready for review and merge @sourmash-bio/devs

[codecov]

Codecov Report

Merging #2159 (07e6576) into latest (2cc6c15) will increase coverage by 7.37%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           latest    #2159      +/-   ##
==========================================
+ Coverage   84.34%   91.72%   +7.37%     
==========================================
  Files         130       99      -31     
  Lines       15320    11044    -4276     
  Branches     2176     2176              
==========================================
- Hits        12922    10130    -2792     
+ Misses       2095      611    -1484     
  Partials      303      303              

Flag	Coverage Δ
python	91.72% <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/sketch/minhash.rs
src/core/src/ffi/mod.rs
src/core/src/index/search.rs
src/core/src/index/sbt/mhbt.rs
src/core/src/sketch/nodegraph.rs
src/core/src/encodings.rs
src/core/src/storage.rs
src/core/src/signature.rs
src/core/src/ffi/signature.rs
src/core/src/ffi/index/revindex.rs
... and 21 more

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[ctb]

thanks!